Volume 5, Number 2, May 1980 A

VOLUME 5, NUMBER 2, MAY 1980

A SNOW INDEX USING 200MB WARM ADVECTION

Billie J. Cook Weather Service Forecast Office Fort Worth, Texas

ABSTRACT location in relation to the maximum. All except Ostby relate heavy Warm advection at 200mb is used to snowfall to the track of the surface forecast the amount of snowfall in cyclone or its upper features. the ensuing 24 hours. The type of advection at 700mb is used to modify 3. TYPE OF PRECIPITATION the forecast. A test of the method was made from October 15, 1976 The type of precipitation must be through March 1, 1977 with favorable determined before snowfall amounts results. are to be successfully forecast. The operational POF forecasts based on 1. INTRODUCTION MOS incorporated the better features of Wagner (7) and Younkin (8). The Most heavy snow forecast techniques MOS POF progs are quite good, and if suffer from being too complicated, they forecast snow any precipitation are too time-consuming, or fail to is likely to be snow. However, a few provide a forecast of both maximum important snowfalls have occurred amount and area. The method when the POF was less than 50 presented here overcomes some of percent. Cooling caused by these difficulties. An initial precipitation is apparently not estimate (or "snow index") is made entirely accounted for in the POF from a calculation using the 200mb progs. Wexler et al (9) show that chart. Adjustments are made based on with a moist adiabatic lapse rate, advection at 700mb and a few simple surface air at a temperature of 40F guidelines. Application of the can be cooled to 32F by .38 inch of technique requires only a few precipitation. Evaporation can minutes. modify the lower atmosphere in a short time. The wet-bulb temperature 2 . BACKGROUND curve from soundings may be used to determine the type of precipitation. Most snow forecasting schemes use the A simple rule is that precipitation location of low pressure centers will be snow or change to snow (surface or aloft), or vorticity whenever the wet-bulb temperature maxima. See Goree and Younkin (1), curve goes below freezing within 2500 Spar et al (2), Hanks (3), Browne and feet of the surface and does not go Younkin (4), and Harms (5). Ostby (6) above freezing at a higher level. suggested the use of Model Output Significant precipitation should be ! Statistics (MOS). Goree and Younkin expected (about .25 inch, water identify prediction of heavy snow as equivalent) . one of the most important functions of the National Weather Service, but In January 1974, the type of state that " ••. on the average, only a precipitation was compared to the modest degree of success can be height of the 200mb surface over the claimed." contiguous 48 states. Snow accounted for 81 percent of the precipitation Most of these approaches make no area between the 164 (Note: Contour attempt at forecasting the amount of values in decameters with initial "1" snowfall. Harms and Hanks make such omitted - e.g., 164 = 11640 meters) an effort but if the area of concern and 176 contours at 200mb. The area is not on the forecast path of was determined by one degree latitude maximum snowfall, then the amount is longitude "squares", where any only an estimate based on the report of precipitation within a

29 NATIONAL WEATHER DIGEST square was assumed to be valid for The thermal field at 200mb can then the entire square. Rain occurred in be a measure of the potential 11 percent and freezing rain in 8 vertical motion over a particular percent of the area. Between the 176 location. For example, if the 200mb and 188 contours snow accounted for temperature at some location is -53C 36 percent of the area, rain 51 and the warmest 200mb temperature percent, freezing rain 12 percent, upstream is -48C, then the potential and ice pellets 1 percent. warming at 200mb as a result of the Experience gained since 1974 suggests upstream system is only 5C and the the 176 contour as the 50 percent vertical motion as it approaches will probability line for low elevations be only moderate. On the other hand, with average surface pressures (1005 if the 200mb t empe r a t ur e at some to 1015mb) • For each 3000 ft l oc a t i o n is -GOC and the warmest increase in elevation or for higher temperature upstream is -45C, then surface pressures the next higher the potential warming at 200mb is 15C contour (188) is used. Also for and the vertical motion will be lower pressures or in the immediate strong, provided the change occurs in coastal sections the 50 percent a reasonably short time. Experience probability is one contour lower has shown a reasonable advective (164) • speed for the warm pocket to be about 14 degrees at latitude (degrees lat) 4. DEVELOPMENT OF TECHNIQUE in 24 hours, or an average of 35 knots. Examination of the 200mb It should be emphasized that these charts over several winter seasons comments are only for wintertime reveals some interesting relations: conditions (mid-October through February). This technique relies A. Cloudiness and precipitation tend primarily on the 200mb chart with to increase under indicated warm minor checks from the 700mb and advection and decrease under surface charts. First a brief word indicated cold advection at 200mb. about the 200mb chart. Warm air normally occurs in 200mb troughs and B. The heaviest snowfall in the cold air in the ridges. Temperatures ensuing 24 hours occurs near the are -40C to -45C in strong troughs coldest air at 200mb that is and are -65C or colder in strong downstream from the warm pocket, ridges. Temperatures remain in the provided of course that the column of -50s with weaker systems. Thus the air is cold enough for snow. This is thermal pattern at 200mb reflects the usually immediately downstream from strength of the system occurring at the strongest indicated warm lower levels. advection at the initial time (T+O).

The indicated advection pattern at C. Vorticity maxima at 500mb tend to 200mb gives important clues to the move toward the 200mb cold pockets; vertical motion fields and, more e.g., if a 200mb cold pocket develops importantly, it indicates the northeast of a 500mb low, the low potential vertical motion field. will recurve in 24 hours. Note that Austin (10) says, "The strong the 200mb warm pocket almost always (temperature) changes which occur in coincides with the 500mb vorticity the upper atmosphere may be maximum, particularly in well considered as arising from vertical developed systems. Generally the motion fields which accompany the direction of movement of a vorticity sea-level pressure changes and which maximum at T+12 to T+36 hours will be arise from such low-level processes parallel to a line connecting the as heating and cooling. A 200mb warm and cold pockets at T+O. non-uniform field of vertical motion On occasions when the 200mb warm creates strong advective fields in pocket is ill-defined or shows as an the lower stratosphere as a result of elongated warm sector, then the 500mb the abrupt change in the vertical vorticity maximum is used as the temperature gradient near the 200mb warm pocket. Forecast tropopause." continued digging if the cold pocket

30 VOLUME 5, NUMBER 2, MAY 1980 is southeast of the 500mb low and the solid arrow, utah to Pennsyl recurvature if the cold pocket is vania, suggested a northeastward northeast of the 500mb low. This movement after 24 hours. Note that technique should not be used with two the track of the 500mb low was types of patterns. They are: large essentially as suggested by the solid scale cyclonic flow over North arrows. America with short waves moving rapidly through, and cutoff lows in D. Cold air (-60C or colder) usually the southwestern U.S. that have develops northwest of a short wave remained nearly stationary for the trough before it deepens. The cold previous 24 hours. air is frequently over the Gulf of The storm that caused blizzard Alaska as the short wave moves on the conditions through Eastern Colorado west coast. and Northwestern Kansas in March of 1977 is traced in Figures la and lb. E. The height contours at 200mb are The crosses connected with the dashed fairly conservative. Extrapolation lines depict the 500mb vorticity of the past 24-hour changes for the maxima. On Figure la points marked next 24 hours usually works well. "B" indicate the 36-hour forecast Generally the contours may be used in barotropic vorticity maxima, while situ or with only a small forecast points marked "L" indicate the change, usually less than 120 meters positions of the 36-hour LFM (one contour interval). vorticity maxima. Dotted lines connect each "B" and "L" to the cross Jacobson et al (11) used the coldest corresponding to the verifying time. temperature at 500mb and the wind flow to forecast surface development. On Figure lb the solid arrows connect Warm air at 200mb reflects cold air the 500mb vorticity maxima with the at 500mb. If the storm is not strong coldest 200mb air downstream. The enough to affect the 200mb head of the arrow marks the central temperature, heavy snow usually does point of the coldest 200mb not occur. Jacobson said it was not temperature. Note the rotation of necessary to worry about the moisture the arrow heads with time. in the developing storms. If the dynamics are strong, the moisture At OOGMT March 11 the LFM forecast a will find its way into the storm. closed low over Northeast New Mexico This appears to be true. Initially - an excellent forecast, while the dry conditions serve only to delay barotropic did rather poorly. In the precipitation a few hours. next three forecast periods the LFM center was too far south while the Oliver and Oliver (12) and many barotropic center was equally too far others concur that cold advection in north. Following these periods both the mid-troposhere (700mb) indicates types of progs improved. The LFM is downward motion and warm advection known to be better than the upward motion. The 700mb advection barotropic in the development stages is used to adjust the forecast but is slow in forecasting derived from the 200mb chart. recurvature. 5. THE INDEX The 200mb temperature pattern seems to give clues about 24 hours in A. The index, or the average advance of the recurvature. Note the snowfall, in inches, for the solid arrow drawn from the 12GMT following 24 hours will be 200mb chart on the 9th indicated a approximately one-half the indicated continued digging through 36 hours. warm advection (C degrees) at 200mb But by OOGMT on the 10th, the solid provided the column of air is cold arrow had turned nearly east-west enough for snow. The maximum reaching from Oregon to West indicated warm advection, limited to Virginia. This suggested the 500mb 840nmi (or 15 degrees lat) upstream low would begin to turn eastward in should be used for a 24-hour about 24 hours. By 12GMT on the 10th forecast. If the indicated warm

31 NATIONAL WEATHER DIGEST

("';v y-1-,-~ , x., \\i ~ \.~/ <: \--:\ !~; :; '-=1":':. B -. --,--'r ~;o''''''''' \ 6'' /I::r---· ! a . \ " V 13' 12Z'T .--J:(J , .'.. ' ;1 , : .: :.,."r:: t\...... -;-...... \I'~ . . , ", : ' 'ti f i: \ ",.I ~ '" . \ , f I : : -.. , ....,rt1 0.0 . J:>~ '. , • : ~O-l2Z : :. ./~ 1 2- 1 2 ~1 \ J A ' : .;." , L j'-r .>:..: ~ '.ll. ...._- 11- 001 , I vf., : : r.:»> il . -~<-2 '+-1- ~? '

~ . depict observed vorticity maxima. Points marked I .«l "B" are 36- hour Ba rot ropic foreca st vorti c it y , ~ ma x ima. Points marked "L" show 36-h our LFM . .__ forecast vort icity max ima . Dotted Iines connect --0--"""' the "Bs" and " Ls" t o th e subsequent ly observed ""_.P position of the vortic it y maxima. \/ !"fl~ (/ ~ t~j "I,,: ~ L' ·r"'~_ .. --r.. ,J .,T:~ '/ . \} ~ i ~ .-I--~-" .... .-r,.,:;, .-t-'-:;-- - ...... f!.",~. :~~'~ ==J:=~~~fj~ffJ~\f· . lr· . · .· ~ r" <>.;;:'>"'.'\ '.• - ~ . " ~;---»: -;1,' 1· r- ~~~~ ·· .' \ or """--.1 r-~ \ .~~\- Figure lb. Crosses same as le , Sol id arrows connect 500mb vorticity maxima with ,. I,'. coldest 200m b air downstream. Head of arrow is I,,:.. .. " positioned at center of 200mb cold pocket.

32 VOLUME 5, NUMBER 2, MAY 1980

33 NATIONAL WEATHER DIGEST advection extends less than 6 degrees of the area of coldest 200mb lat (360nm) upstream from the temperature, or limited to 14 degrees forecast area, the precipitation is lat downstream from the warmest 200mb usually of short duration. Generally air for a 24-hour forecast (21 the indicated warm advection is degrees lat for a 36-hour forecast). measured from the warmest temperature to the coldest temperature along the The snowfall of December 6-7, 1976 is 200mb contours. presented as an example of an average snowfall event (Figure 2). It was B. If cold advection is observed selected because a forecast was made within 8 degrees lat of the forecast shortly after receiving the 200mb area at 700mb, the snowfall forecast chart for OOGMT December 6, 1976. in paragraph A should be divided by The dotted line outlines a forecast 2. The index then becomes 1/4 the area for an average of 2-1/2 inches indicated warm advection at 200mb. of snowfall to verify at l2GMT The 700mb temperature 8 degrees lat December 7, 1976. Warm advection was upstream shoUld be compared with the indicated at 700mb using the criteria 700mb temperature over the forecast described earlier. Five degrees warm area. If the upstream temperature is advection was indicated at 200mb from colder than the forecast area Colorado to the forecast area giving temperature, then cold advection i s a snow index of 2-1/2. The area was assumed and the 1/4 rule is used; selected as being in a cold 200mb otherwise the 1/2 rule is used. temperature zone which was downstream from the warmest 200mb temperature. It was selected in an area where the C. Maximum snowfall in the ensuing POF was 50 percent or higher for 24 hours occurs near the coldest three subsequent l2-hour forecast 200mb temperature that is downstream periods. The western end of the area (parallel to contours) from the was placed slightly downstream from warmest 200mb temperature. It will the significant warm advection and frequently be within or near the area the eastern end was placed far enough outlined by Ostby (6). Heavy snow to cover most of the cold 200mb bands are almost always parallel to temperature. Twenty-one latitude the T+O 200mb contours or cross them degrees downstream from the warm at a slight angle from higher to pocket allows for a 35 knot average lower values as the snow progresses, movement through the 36-hour forecast usually northeastward. This is the normal turning of the 200mb contour period. The width of the forecast pattern as the storm moves area was selected as 120 meters on northeastward. the 200mb chart. The observed 24-hour snowfall area at D. Local effects (e.g., the Great 12GMT December 7, 1976 is shown by Lakes, hills and mountains) can the dash-dot line. It includes all affect the amount of snowfall reports of more than one inch. The considerably. average in the area was 2-1/2 inches, with a maximum of 4 inches. This E. Location of the area of heavy forecast was considerably better than snow is usually the main problem. As average and took less than 5 minutes noted previously the heaviest snow is to prepare. The surface chart for almost always immediately downstream OOGMT December 6, 1976 is shown in from the maximum indicated warm Figure 3. Twenty-four hours later advection on the 200mb chart. The the occluded front extended from a width of the heavy snow band is weak low north of the Great Lakes frequently about the same as the through lower Michigan to another distance between adjacent contours on weak low in southern Louisiana. the 200mb chart. The southwest end of the heavy snow band should be placed just downstream from the maximum indicated warm advection. 6. TEST OF TECHNIQUE The northeastern end of the forecast area should be near the eastern side The snow cover as indicated on NAFAX

34 VOLUME 5, NUMBER 2, MAY 1980 map N73 was used to determine the 1. Heavy snow should not amount of snowfall for each day from be forecast beneath a October 15, 1976 to March 1, 1977 by strong confluent 200mb subtracting the snow depth for the flow. The heavy snow will previous day from that for "today" usually be south of the over the 48 states and adjacent areas confluent zone. of southern Canada. The liquid precipitation indicated on NAFAX N81 2. Heavy snow should not was a lso used to "confirm" the be forecast south of the snowfall. This method has obvious surface low. Occasionally weaknesses, notably the assumption the 200mb warm pocket and that each snowfall began after 12GMT cold pocket will appear and ended by 12GMT the next day. south of a well-developed Settling and melting are not surface low. Under these accounted for and surely some large conditions l ittle or no amounts are omitted from the snow precipitation occurs, as cover chart. Nevertheless, the size the surface low . usually of the sample and the cross checking moves rapidly eastward, with the liquid precipitation should parallel to the 200mb have given a fair estimate of the contours. This situation average snowfall. Mountain top occurs most frequently wi t h reports and isolated reports, as well northwest flow aloft. as reports obviously influenced by the Great Lakes, were not used in 3. Under northwest flow this test. A minimum of three aloft the systems can move reports over an area of at least 3000 rapidly and some snow may square miles was required for fall with indicated cold consideration as a snow event. advection at 200mb. This was observed five times The highest index obtainable in the during the test period. U.S. or adjacent section of Canada Maximum snowfall for these was derived from the 12GMT charts events was two inches. each day during the test period and Most stations reported 1 compared to the snowfall reported 24 inch. hours later. On 58 days no index was recorded due ~o a lack of significant 4. Occasionally a weak 200mb warm advection; no significant trough-ridge couplet at snow was reported on the following 200mb will accompany a days. All other indices and 500mb low and signal a turn subsequent snowfall are listed i n eastward too early. This Table 1 and compared in Figure 4. pattern has very weak f l ow The abscissa is the snow index (or between the trough and forecast snowfall) and the ordinate ridge and the half-wave is the observed average snowfall. length is usually l es s than The plotted numbers represent the 10 degrees lat. maximum snowfall reported. A perfect forecast of the average snowfall 5. This forecast scheme is would fallon the heavy line. The known to be unreliable in l ighter lines above and below spr ing and early fall. It represent a departure of one inch. should not be us ed before There were a few occasions where one October 15 or after March or more of the charts were missing, 10. Late spring s t o r ms and more than one snow e ve nt was have been observed with r eco r ded o n a few charts. Of the 80 little temperature s now events, 64 percent were within advection at 200mb and o ne inch of t he forecast amount. others have occurre~ under the strongest 200mb warm advection rather than the Here are some of the known weaknesses coldest air, as used in of this technique: winter.

35 NATIONAL WEATHER DIGEST A series of 200mb charts is presented The 200mb chart for 12GMT January 29 (Figures 5 through 8) to illustrate (Figure 5) indicated the strongest the sequence of events in connection warm advection over southern Arizona. with heavy snow which fell over the Maximum precipitation was reported northern portions of Texas and over southern Arizona at l2GMT Louisiana in late January 1977, as January 30. A strong confluent zone delineated by the dash-dot line in was noted from the four corners to Figure 7. Cold air had pushed the Texas Panhandle. The coldest air southward with the surface cold front was from southeastern New Mexico well into the Gulf of Mexico by l2GMT across North Texas, a potential January 29, 1977. A 500 mb closed threat zone for significant low off the southern California coast precipitation. was forecast to move eastward while opening into a trough by l2GMT By OOGMT January 30 (Figure 6) January 31, 1977. temperatures warmer than -50C were

Table 1 Observed Observed Average Maxillur:l Av('r_'J~c Maximw! IlIi& Index Snowfall Sn~fall Location Date Index Sno"t;111 Snowfall Location lO~18·76 -2 - 2 ERN SD-MN 17-27-76- 2- -2 -3 loIN-HI 10-20-76 2 • 8RN QUE 12-28-76 1.5 2 • KY 10-21-76 2.S • SRN QUE 1-1-77 3 1 3 TX PNHDL-SRN KS 10-22-76 2.S • MT 1-2-77 2.5 3 $RN MO-WRN lY 10·23·76 I 2 2 ERN SD-)Qi 1-2-77 3.S 5 OR-ID 10-26-76 2.S • • ERN CO-WRN K5 1-3-77 • 3 5 NV 10-31-76 S S 11 SERN QUE 1-3-77 2.5 3 • NY 10-31-76 '.S S S VT-NIl 1-4-77 '.5 4 6 NRN NO 11-3-76 3 I I NIl 1-4-77 4 • WV 11-5-76 • 2 S NY·NlI 1-7-77 3 6 • NA-ME 11-6-76 I.S 2 2 LWR HI 1-8-77 i.s 3 5 TX PNHDL 11-11-76 a 2 S KY-WV 1-9-77 13 &RN OK-SRN HO-NRN AR 11-12-76 • • • SERM mt-WRN TX 1-9-17 ? IL-OH 11-13-76 • • 10 N CNTRL TX 1-11-77 2.5 • ID PNHOL 11-13-76 3 3 S OWRN AR 1-12-77 NEG 2 NO 11-14·76 3 2 3 WV 1-13-77 2.5 3 IN-OH 11-15-76 I.S o o cr 1-14-77 PA 11-11-76 I.S 3 • SRN ONT l-lS-77 NEG ERN MJ-IL 11-19·76 I I I NY-MA 1-17-77 I NS 11-24-76 2 I I NERN MT 1-18-77 o AL·GA 11-25-76 3 2 S MT-NY 1-18-77 NEG I SERN NO-NWRH IA 11-26-76 3 2 S WRN I

36 VOLUME 5, NUMSER 2, MAY 1980 analyzed across southern and lower ness forecasts indicated the snow California while the -GOC isotherm would be north of the 200 contour on continued from eastern New Mexico the 200mb chart. Radar detected eastward across the southern states. scattered light rain over extreme The strong confluent zone was across West Texas beginning at 05GMT. northern New Mexico, the Texas Precipitation changed to snow within Panhandle, and northern Oklahoma. a few hours. Strong warm advection outside the confluent zone was occurring across By l2GMT January 30 (Figure 7), the northern Mexico into southeastern New strongest indicated warm advection at Mexico and was forecast to continue 200mb was l7C over northeast Texas. eastward across North Texas. The Warm advection was continuing at 700mb temperature 8 degrees lat 700mb, the index became 8-1/2 inches. upstream was 5C warmer than that The strong confluent zone continued observed over North Texas. The snow ov~r northern Oklahoma and, to a index was therefore computed as 10 lesser extent, over northern degrees warm advection at 200mb, Arkansas. The first indication of a divided by 2 to equal 5 (inches of surface low was a reported 1017mb snowfall) • Observations and thick- pressure near Brownsville, 330nm 8

~~6 10~~

~~5..~~

~~'. 5.~~

~~. . o 4 5 6 7 8 SNOW INDEX~~

Figure 4. Graph of snow index vs. snowfal I. Abscissa Is snow index, or forecast snowfall In Inches; ordinate is observed average snowfal I in 24 hours. Plotted points sized proportional to number of cases. Numbers plotted for each point represent reported maxima.

~~37 NATIONAL WEATHER DIGEST~~

~~Figure 5.~~

~~2'l.._--pJ;="",..,:,J.,;....:..--::~r~ Figure 6. 200mb~~

~~38 VOLUME 5, NUMBER 2, MAY 1980~~

~~.'"'..?~~