# WEATHER ANALYSIS & MIDLATITUDE STORMS Summer 2004

Date Due: Wednesday, 4 August 2004

The total maximum points were  100.  Point distribution for each question is noted below.

## PART #5.1. SURFACE WEATHER MAP ANALYSIS

The total maximum points were 100 for Part #5.1 and 50 points for Part #5.2.

The attached map contains the surface weather features observed over the upper Midwest at 6:00 AM CST on Tuesday, 10 November 1998.

## A. STATION MODEL

At each station the following abbreviated surface station model has been utilized:

Wind arrows fly with the wind. Each full barb equals 10 knot wind speed. [Sample station reports a 15 knot wind from the southeast (SE)]. Missing values are indicated by an " M".

What were the conditions reported at Madison, WI at map time? (Include units)

 Temperature:  Dewpoint:  Pressure:  Sky cover:  Wind Speed: Wind Direction: Precipitation type or significant weather phenomenon: 53 degrees F  51 degrees F  986.6 mb  Overcast (10/10 clouds)  15 knots (Since part of wind flag was obliterated, an answer of 5 -10 knots was accepted) Southeast  Steady Rain

## B. ISOBAR ANALYSIS

Remember that isobars portray the sea-level corrected pressure field. Analyze the sea-level pressure field by drawing isobars spaced every 4 mb, centered upon 1000 mb (i.e., ..., 992, 996, 1000, 1004, 1008, ..., etc.). The reported values may not necessarily equal the desired values. Therefore, you would have to interpolate between those stations with pressures within + 4 mb of the desired value. When performing your analysis make a light sketch of the isobars with a pencil initially; then smooth the isobars. After smoothing and checking your analysis, isobars may then be inked with a black pen and each labeled neatly.

Points to consider during your isobar analysis:

1. Where is the low or high pressure center(s) located?

(Identify the pressure feature and state or province)

 The surface low was in northwest Iowa, near Mason City.  The surface high was in the Quebec - off the map

Hint: you should look for the regions with a distinct, closed wind circulation. Locate the center of the low or high pressure system and mark with a large red block "L" (for the low) or a large blue "H" (for the high). Give an estimate of the value of the central pressure.

 The central pressure of the low over Iowa was approximately 970 mb.

2. Since the atmosphere is a continuous fluid, no discontinuities nor sharp kinks should appear in your isobar analysis. The spacing between isobars should suggest a smooth horizontal pressure gradient, if possible. Some kinks in the isobars should be expected, especially near wind shift lines and fronts.

3. The isobars are usually spaced closer in regions where the wind speed is greatest.

4. Does your smoothed analysis conform to the above items ?

 Yes it worked for me!

## C. ISOTHERM ANALYSIS

Analyze the surface temperature field by drawing isotherms every 10&deg; F (e.g., 10&deg; , 20&deg; , 30&deg; ,...). Some smoothing may be necessary. Attempt to follow the same rules as for drawing isobars noted above; however, the temperature field is usually more variable than the pressure field. The finished isotherm analysis should be drawn in dashed red lines and each isotherm should be labeled accordingly.

Points to consider during your isotherm analysis:

1. Where are the warmest and coldest regions (by state) found?

 a. Warmest Southern Indiana, Illinois or western Kentucky.  b. Coldest Montana and Wyoming.

2. Where are the sharpest horizontal temperature contrasts located?

 West of the Mississippi River, extending from central Iowa (near the low) south through central Missouri.

## D. INTERPRETATION OF THE SURFACE ANALYSIS

1. Where are the regions of a) highest and b) lowest dewpoints?

 a) Highest (humid) Kentucky, Tennessee and southern Illinois  b) Lowest (dry) Wyoming and Montana

2. What type of precipitation is found in:

 a) Illinois Steady Rain  b) Nebraska Steady Snow  c) North central Iowa - Southeast Minnesota Rain and one Iowa station also reported a thunderstorm

3. What is the prevailing wind direction that is observed in the following locales?

 a) North Dakota = North-northeast winds  b) Central Illinois = South winds

4. In what region do you find the strongest winds on the map?

 South Dakota (also Nebraska is acceptable)

Are the isobars closer or farther apart in this area?

 Yes, they are closer to the west of the low center.

5. What region(s) is experiencing warm advection , where winds are transporting warm air toward colder regions?

 Illinois and South-central Wisconsin

What region(s) is experiencing cold advection, where winds are transporting cold air toward warmer regions?

 Missouri and essentially most of states in region that are west of Mississippi River.

## E. FRONTAL ANALYSIS

With the aid of your isotherm analysis, the distribution of dewpoint temperatures and the wind field through the regions of distinct wind shifts, can you identify a cold front? A warm front? Using the conventional frontal symbols (see Figure 1.3 on page 4 of your text), draw the appropriate surface fronts on your analysis.

## PART #5.2 WEATHER SEQUENCES & FRONTAL PASSAGES

For each of the weather elements listed, describe the time sequence that you would expect to observe at your "weather station" during:

1. A warm frontal passage in autumn resulting when an idealized cyclone passes from south to north, while staying west of you:

 `PRE-FRONTAL` `FRONTAL` `POST-FRONTAL`
 ` CLOUDS (type)      Cirrus/cirrostratus                                    to altostratus                                   to nimbostratus/stratus to fog to    clear` ` PRECIPITATION       Steady rain to drizzle 'till frontal passage  then  none         (type)                              ` `TEMPERATURE (trend)           Cool                         to  Warm` `WINDS (direction) Northeast --->East -----> Southeast to south ` ` PRESSURE TENDENCY   Falling              Unsteady  Steady to slow rise`

2. A cold frontal passage in the spring of the year when an idealized cyclone moving eastward passes from the northwest to the northeast of you:

 `PRE-FRONTAL` `FRONTAL` `POST-FRONTAL`
 ` CLOUDS (type)      Clear to Cumulus to Cumulonimbus  to clear  and stratocumulus` ` PRECIPITATION        Rain (snow) Showers    Thunderstorms     Snow (rain) showers(type) ` `TEMPERATURE (trend)           Warm                            to     Cold` `WINDS (direction) South Southwest West Gusty Northwest ` ` PRESSURE TENDENCY   Falling                   Unsteady    Rapid rise`

3. The passage of an idealized snow producing cyclone (and attendant trough line) in winter traveling from southwest to northeast, but remaining just south of you:

` `
 `BEFORE TROUGH` `TROUGH PASSAGE` `AFTER TROUGH`
 ` CLOUDS (type)    Cirrus/cirrostratus to altostratus to nimbostratus to stratocumulus` ` PRECIPITATION        light snow to heavy snow  ---> decreasing to snowshowers(type) ` `TEMPERATURE (trend)            Cool    ------->                  to ------>            Cold` `WINDS (direction)   Northeast -----> North ---------> Northwest ` ` PRESSURE TENDENCY   Falling   ----->                steady ------->       Rising`

Latest revision: 5 August 2004 (0200 UTC)

Produced by Edward J. Hopkins, Ph.D.
Department of Atmospheric and Oceanic Sciences