December 5-6, 2013 Lab Exercise

Objective:

To become comfortable with:

• Interpreting winter weather forecast products before an event (Part 1) • Monitoring key winter weather ingredients and products during an event (Part 2)

Instructions:

Answer the questions in this lab exercise in the space provided as best you can. You may work on your own or in a group (no more than 6 per group). You are welcome to use any materials from class to help you complete the exercise.

Area of Focus:

For the purposes of this lab exercise we will monitor conditions in the blue shaded portion of Oklahoma. The boundaries of the different National Weather Service offices that cover Oklahoma are shown in black.

Tulsa Amarillo NWS NWS Norman NWS

Shreveport NWS

1 Part 1: Interpreting Winter Weather Forecast Products Before an Event

It is the morning on Monday, December 2, 2013 and you have been hearing about the possibility of a major winter storm impacting Oklahoma towards the end of the work week. You take a quick look at the available graphicasts from the Norman and Tulsa National Weather Service (NWS) offices.

Question 1. Based on the information shown in these graphicasts what is known as of Monday morning? What is less certain?

2 It is now the morning of Tuesday, December 3, 2013 and you need to get more details on the forecast so you check out the Area Forecast Discussion from the Norman and Tulsa NWS offices.

Norman Area Forecast Discussion (Issued 4:19 AM Tuesday, Dec 3, 2013)

RELATIVELY WARM AND HUMID WEATHER WILL CONTINUE TODAY AND TONIGHT...AS A MUCH COLDER AIRMASS BEGINS ITS PLUNGE DOWN THE PLAINS TO OUR NORTH. THE INITIAL WIND SHIFT WILL SWEEP THROUGH OKLAHOMA AND NORTH TEXAS THIS EVENING INTO TOMORROW MORNING. THE MAIN COLD FRONT WILL FOLLOW BEHIND THE INITIAL SHIFT BY ABOUT 12 HOURS.

AT FIRST...A RAPID COOLING AND STRONG NORTH WINDS WILL BE THE PRIMARY EFFECTS OF THE NEW AIRMASS. HOWEVER...BY THURSDAY MORNING...THERE WILL PROBABLY BE ENOUGH MOISTURE ABOVE THE COLD AIRMASS TO START SOME LIGHT PRECIPITATION. INITIALLY...MOST OF THE PRECIPITATION SHOULD BE RAIN OR FREEZING RAIN...DEPENDING ON SURFACE TEMPERATURES. AS TIME PROGRESSES...AND EVER-STRENGTHENING UPPER-LEVEL SYSTEMS PASS OVERHEAD...THE PRECIPITATION WILL BECOME MORE LIKELY...AND HEAVIER. IT WILL ALSO CHANGE FROM RAIN...TO FREEZING RAIN...TO SLEET...TO SNOW...IN A PROGRESSION FROM NW TO SE.

THERE IS NOW ENOUGH CONFIDENCE THAT SOME KIND OF SIGNIFICANT WINTER WEATHER EVENT WILL OCCUR THAT WE HAVE ELECTED TO ISSUE A WINTER STORM WATCH FOR THE AREAS MOST LIKELY TO BE AFFECTED BY EITHER HEAVY SNOW...HEAVY SLEET...SIGNIFICANT ACCUMULATIONS OF ICE...OR SOME COMBINATION THEREOF.

MODELS CURRENTLY SUGGEST A BAND OF RELATIVELY HEAVY SLEET AND SNOW FROM NEAR LAWTON TO OKLAHOMA CITY TO CUSHING LATE THURSDAY AND EARLY FRIDAY. ACCUMULATION AMOUNTS WILL BE HIGHLY DEPENDENT ON THE RATIO OF SLEET TO SNOW...AND THE AMOUNTS IN THIS FORECAST ASSUME A FAIR AMOUNT OF SLEET. THE MODELS ALSO INDICATE THE POSSIBILITY OF AN ICE STORM ACROSS SOUTHERN OKLAHOMA INTO WESTERN NORTH TEXAS...MOST LIKELY SOUTHEAST OF A LINE FROM ARDMORE TO HOLDENVILLE...BUT PERHAPS A LITTLE FARTHER TO THE NORTH AND WEST. OBVIOUSLY...WITH THE EVENT STILL TWO TO THREE DAYS OUT...THE DETAILS ARE LIKELY TO CHANGE. IN ANY CASE...IT IS LIKELY THAT SOME PART OF THE WINTER STORM WATCH AREA WILL RECEIVE SIGNIFICANT SNOW...SLEET...AND/OR ICE ACCUMULATIONS.

Tulsa Area Forecast Discussion (Issued 4:44 AM Tuesday, Dec 3, 2013)

WINTER STORM WATCH ISSUED WITH THIS FORECAST CYCLE. 18Z THURSDAY THROUGH 00Z SATURDAY CHOCTAW PUSHMATAHA LEFLORE SEBASTIAN FRANKLIN COUNTIES. REST FROM 12Z THURSDAY THROUGH 00Z SATURDAY.

ARCTIC COLD FRONT ARRIVES WEDNESDAY TO SET THE STAGE FOR AMPLE COLD AIR BELOW ABOUT 850 LEVEL. MEANWHILE WARM LAYER REMAINS THROUGH FRIDAY FOR PROLONGED MIX PRECIP EVENT. ALL MODEL TEMP PROFILES INDICATE LARGE SUB- FREEZING LOWER LAYER WHICH COULD PRODUCE MORE SLEET THAN THIS AREA HAS SEEN IN SOME TIME. PRECIP WILL COME MAINLY IN TWO DISTINCT ROUNDS WITH ABOUT A 4 TO 8 HOUR BREAK IN BETWEEN PRECIP OR JUST TRACE AMOUNTS DURING THAT PERIOD. THIS BREAK NECESSITATED AN UNUSUALLY LONG WATCH PERIOD.

CONFIDENCE HIGH IN COLD AIR AND MIXED PRECIPITATION. CONFIDENCE ALSO RATHER HIGH THAT A MAJORITY WATCH AREA WILL BE CONVERTED TO A WARNING AT SOME POINT. CONFIDENCE NOT AS HIGH REGARDING TIMING / TRANSITION FROM ONE PRECIP TYPE TO NEXT. THUS FORECAST SNOW ICE AMOUNTS AT THIS TIME SHAKY. AREAS NEAR/NORTHWEST OF INTERSTATE 44 PRIMARILY SLEET-> SNOW WHERE FOUR PLUS INCHES SNOW/SLEET ACCUMULATION LIKELY. FAR SOUTHEAST OK INTO WEST CENTRAL ARKANSAS MOST LIKELY TO RECEIVE SIGNIFICANT ICE ACCUMULATION FROM FREEZING RAIN. AND EVERYTHING GOES BETWEEN I-44 AND SOUTHEAST ZONES.

UNSEASONABLY COLD BEHIND THIS WINTER MESS WITH COLD WEATHER LASTING WELL INTO NEXT WEEK. CHANCE OF LIGHT SNOW SUNDAY AS THE UPPER TROUGH NEARS.

Question 2. A lot of information is contained in the above Area Forecast Discussions. List 3 key points you took away from these discussions. For reference, a map containing advisory information mentioned in the discussions is shown to the right.

Winter Storm Watch

3 It is now Wednesday, December 4, 2013 and you view several Day 2 NWS Weather Prediction Center products including Snow Accumulation (top image) and Ice Accumulation (bottom image).

Question 3. In the top image what does it mean for areas within the blue line? The green line?

Question 4. In the bottom image, what does it mean for areas within the blue line? The green line? The red line?

4 It is now the early morning of Thursday, December 5, 2013. You do a final check of various graphical products shown below including an advisory map, graphicasts from Norman and Tulsa, and WPC forecasts.

Question 5. What aspects of the forecast have changed since you first started watching it on Monday? In your opinion which of the following product(s) did you find to be the most helpful leading up to the storm?

Weather Advisories Map

Winter Wx Advisory

Winter Storm Warning

Ice Storm Warning

Norman NWS Graphicast Tulsa NWS Graphicast

WPC 4 inch Snow Accumulation Probability WPC 0.25 inch Ice Accumulation Probability

5 Part 2: Monitoring Key Winter Weather Ingredients and Products During an Event

Question 6. What 2 “weather ingredients” are needed for winter storm events?

Question 7. A 500 millibar map from 6AM on December 5, 2013 is shown to the right. What is the name of the feature west of Oklahoma? Vertically speaking, what do these features produce on their eastern side?

The 6AM December 5, 2013 sounding (weather balloon) from Norman, OK is shown below. The blue line is the 32F line, so anywhere to the left of it is below freezing.

Question 8. There are two thick black squiggly lines on the below sounding – label each of them. What do we call the feature that sticks out to the right of the blue line? If precipitation occurred in central Oklahoma right now, what precipitation type do you think it would be?

6 Precipitation rapidly increases midday on December 5, 2013. You view combination maps of Oklahoma Mesonet (air temperature and wind) overlaid with Base Reflectivity (OKC and Tulsa) throughout the duration of the event.

Question 9. At 2:40PM what do you suspect the primary precipitation type is from SW Oklahoma up into central Oklahoma? Why?

Question 10. At 5:30PM what do you suspect the primary precipitation type is in eastern Oklahoma (south of Tulsa)? Why?

7 Question 11. At 11:30PM what do you suspect the primary precipitation type is in southeastern Oklahoma (e.g., Marshall, Bryan, Atoka, Pushmataha, and Choctaw counties)? Why?

A new round of precipitation forms early the next morning (December 6, 2013). You check out the 6AM Norman sounding below (left image) as well as a combination map of Oklahoma Mesonet and Base Reflectivity (right image) at the same time.

Question 12. At 6AM (December 6, 2013) what do you suspect the primary precipitation type is throughout Oklahoma now? Why? How did the sounding change since the one you saw 24 hours ago?