Investigation 7A

DataStr= eme Atmosphere Current Weather Studies 7A:

PRECIPITATION PATTERNS

At the end of last week and over the weekend several areas of the country experienced precipitation. Portions of all three coasts, Pacific, Gulf of Mexico and East Coast, had rainfall while interior sections of the Northeast received significant snow. We'll consider the progression of these systems across their respective areas of the count= ry.

Image 1 is the surface weather map for Friday morning, 12Z 16 OCT 2009. A low-pressure center with an associated cold front was approaching the North= west coast. An extensive frontal system was stretched across the Gulf coast and extended into the Atlantic. And the oval curvature of isobars off the New England= /Mid Atlantic coast was associated with a Low that had moved northeastward along= the coast earlier and was centered just east of the map area. Shadings on the m= ap denoted where a national network of NWS radars were detecting targets, typically precipitation (rain and/or snow). Light blues and greens on-screen were typical of light rain or snow while embedded red speckles like those f= rom Florida to northern Mexico and off Washington state generally signified heavier showers, often thunderstorms. The radar shading scale was shown along the left edge of the= map area.

1= . One fairly extensive patch of precipitation was spread over New England to Pennsylvania and bordering states. Winds there were generally from the north or northeast. T= he wind directions [(did)(di= d not)] fit the expected circulation pattern around low-pressure centers as given by the hand-twist model. The northeasterly wind directions were associa= ted with the storm centered offshore, thereby being termed a nor'easter.

2= . This wind flow was part of the general circulation pattern that brought humid air toward the higher terrain in the northeastern U.S. from [(the Atlantic Ocean)(<= u>the central U.S.)].

3= . Along with the moisture supply, the lifting mechanism would have been [(= frontal)(orographic= )].

4= . Also at map time, the frontal system in the Southeast was shown as stationary in= the Atlantic connected to a (partially obscured) Low center in southeastern Georgia= . From the Low, the front was depicted westward as a cold front to southern Texas and then stationary again back up to New Mexico. The stationary front in the Atlantic bent ar= ound to connect to the low center of the nor'easter beyond the edge of the map a= rea.

Radar echoes identified precipitation spread = from South Carolina to southern <= st1:place w:st=3D"on">Texas across the Gulf coastal region. = In the eastern Gulf of Mexico across to the Atlantic, this precipitation generally fell to the [(north)(south)] of the Georgia Low. Red sh= adings (on screen) within the area indicated heavier showers and thunderstorms.

5= . Precipitation was also scattered [(along)(= well to the south of)] the western Gulf cold front.

6= . Comparing the temperature (79 °F) and dewpoint (75 &d= eg;F) at Tampa, FL south of the front, with the the temperature (52 °F) and dewpoint (48 °F= ) at Jackson, MS north of the front, indicates that the air was [(warmer and more humid)(cooler and less humid)] northwest= of (behind) the front.

7= . As the cold front advanced southeastward, the denser air behind the cold front= [(would)(would not)] have acted as the lift= ing mechanism for the humid air to produce precipitation.

8= . Another extensive area of precipitation [(was)= (was not)] located over western Oregon and Washington state along the coast. This area has mountains oriented generally north-sou= th along the Pacific coast. This orographic precipitation was the result of winds circulating around the Low center and ahead of the cold front against rising terrain.

9= . The circulation of air around this northwestern Low [(would)(would not)] have brought air from = off the Pacific Ocean. Fairly widespread are= as of precipitation require atmospheric moisture and a means to cause air to rise leading to adiabatic cooling, saturation and prec= ipitation, as discussed in the Week 6 investigations. All the precipitation areas of t= he country noted displayed these conditions.

1= 0. Image 2 is the surface weather map for Friday evening, 00Z 17 OCT 2009, twelve hours following the Image 1 map. The nor'easter had moved northeastward and could no longer be evidenced by the isobar pattern. The Georgia Low had spl= it with two centers shown by Ls, one lingering in northeast GA and one along t= he front in the Atlantic. The front in the Northwest had reached the Washington state and British Columbia coasts.

The wind flow, as shown by the north and nort= heast winds across New England, obviously [= (did)(did not)] maintain the conditions = for precipitation over that region.

1= 1. The precipitation in the Northwest covered a [(similar)(very different)] area as the Image= 1 map. Comparing the two maps, the forcing mechanisms and moisture sources for the precipitation areas indicated by the radar echoes remained essentially = the same over this time period.

1= 2. Image 3 is surface weather map for Saturday evening, 00Z 18 OCT 2009, one day following the Image 2 map. The original nor'easter had departed the area to= be replaced by the Lows derived from the Gulf coastal system. The remains of t= he northeastern GA Low was now located off the Chesapeake Bay while the low on= the frontal system was well into the Atlantic off Georgia. The front had cleare= d Florida and was into the Florida Strait and over we= stern Cuba. Meanwhile, in the Northwest, one Low center remained along the coast with an occluded/cold front curving inland, through Washingt= on and Oregon and back into the Pacific over northern California. A stationary front also separated milder Pacific air to the southwest from = cold Canadian air that settled in the central portion of the country following t= he eastern storms and brought a flurry of low temperature records over the weekend.

Relatively extensive precipitation remained o= ver the Middle Atlantic states. Based on the probable moisture source and circulation patterns indicated, [(similar<= /i>)(very different)] processes from ea= rlier ones caused the rainfall in this second nor'easter of the weekend.

1= 3. The atmospheric moisture supplied to the frontal system across the Northwest was from the Pacific and the flow following the cold front was onshore, toward mountainous terrain. This precipitation mechanism [(did)(did not)] remain as was generally = shown on the Image 1 and 2 maps.

This series of maps shows the existence of precipitation areas in several sections of the country derived from the combinations of atmospheric moisture sources and broad scale lifting mechanisms. The period of the precipitation results from the movement of the causes, such as the fronts, or the persistence in an area where the causes (e.g., terrain, wind direction) either do not move or continue to be reinforced.

The Applications section of Investigation 7A in the Weather Studies I= nvestigations Manual displays another case of a vigorous front producing precipitation along the Southeast Coast from the perspectives of detailed regional observations and satellite water vapor fl= ows.

Place the answers to Current Weather Studies activities 7A and 7B on the= CWS Answer Form (provided from the DataStreme Atmos= phere website on Wednesdays).

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