221 
fxus62 krah 211658 
afdrah 


Area forecast discussion 
National Weather Service Raleigh NC 
1158 am EST Sat Nov 21 2009 


Synopsis... 
a low pressure system in the northwest Gulf of Mexico will move 
slowly northeast toward the Florida Gulf Coast tonight...before 
weakening and reforming along the Carolina coast Sunday night. This 
will bring increasing cloud cover and a good chance of rain to 
central North Carolina by Sunday night. 


&& 


Near term /through Sunday/... 
as of 1200 PM Saturday... 


16z surface analysis showed 1024 mb high pressure centered over OH/WV. 
1010 mb low pressure was centered near buoy 42361 south of la. 
Aloft...12z radiosonde observation analysis showed a split flow pattern across the 
Continental U.S.. a southern stream 500 mb low was centered over southeast Texas. 
A 90 knots southern stream jet was noted along the southern periphery 
of the upper low over northern Mexico and the northern Gomex. A 100 
knots northern stream jet was noted over the northern middle-Atlantic 
ahead of a deamplifying northern stream trough over the lower Great 
Lakes. A ridge aloft was present over the central and northern Great 
Plains. 


Today: 
the upper low currently over southeast Texas is prognosticated to move east-northeast 
into la by 00z this evening...while the associated surface low currently over 
the northwest Gomex is prognosticated to be located just south of the la 
coast by that time. Surface high pressure is expected to remain centered 
over portions of the mid-Atlantic...lower Great Lakes and Ohio Valley 
in associated/west confluent flow aloft...with a surface ridge axis extending 
south into the Carolinas. Expect a steady influx of middle/upper level 
cloud cover throughout the day today as moisture moves downstream of 
the upper low along the Gulf Coast into the southeast states and 
Carolinas. Moisture advection will generally be limited to the upper 
levels today...and cloud bases will remain at or above 15-18 kft. Lift today 
will be either very weak or non-existent across central NC. Although 
there is currently a northern stream jet entrance region north/northwest of 
central NC...this feature is prognosticated to weaken and lift northeast 
away from the area. Model isentropic fields do indicate increasing 
isentropic lift across the southeast states this after/evening as a 
925-850 mb warm front moves northward toward the central Gulf Coast 
ahead of the surface/upper low in la. However...isentropic lift will be 
weak and confined well south/SW of central NC. As a result...expect 
no chance for precipitation this aft/eve. High temperatures will be complicated 
primarily by broken skies associated/west upper level cloud cover. 
Thicknesses would argue for highs in the ~63f range with full sun... 
however...given weak low level northeast flow and broken upper level 
cloud cover...a range of 60-63f looks more reasonable...although 
even that could be warm by 1-2f depending on the opacity of cloud 
cover. 


Tonight: 
the upper low currently over southeast Texas is prognosticated to deamplify 
tonight as it moves slowly east-northeast into the deep south. The attendant 
surface low is also prognosticated to weaken a few millibars as it moves along 
a baroclinic zone located over the Central/Northeast Gulf Coast. 
Despite the weakening southern stream dynamics...isentropic lift is 
expected to strengthen over the southeast states into the southern 
Carolinas tonight as a 925-850 mb warm front lifts northward into 
Alabama/Georgia ahead of the low pressure system along the Gulf Coast. The 
best low level moisture and isentropic lift is expected to remain 
well S/SW of central NC through 12z sun...with precipitation chances 
largely confined to Alabama/Georgia and perhaps southern/western portions of 
SC. As a result...expect a dry forecast to continue through the 
overnight hours. Middle/upper level cloud cover will be pervasive 
across the area tonight in associated/west the low pressure system upstream 
of the area...and that will be the primary complicating factor 
west/regard to low temperatures. A MOS guidance blend looks pretty 
reasonable...with lows in the Lower/Middle 40s. 


Sunday: 
latest 12z model guidance continues to show the 500 mb low 
deamplifying into a shortwave during the day Sunday as it moves north/NE 
into northern Alabama/Georgia and eastern Tennessee. The attendant Gulf Coast surface low 
is prognosticated to weaken as it remains centered near the Florida/Alabama coast. An 
inverted surface trough/coastal front is expected to sharpen off the 
southeast/Carolina coast during the day sun in associated/west weak upper 
forcing associated/west the deamplifying southern stream wave. Surface high 
pressure over the middle-Atlantic is prognosticated to strengthen and shift 
northeast into New England on Sunday...with a prominent surface ridge 
axis extending south into the Carolinas. Although upper level lift 
will be rather anemic Sunday...isentropic lift in the lower levels 
will be strengthening from the south/SW throughout the day as the 
925-850mb warm front stalls out just to the south in SC and the 
mslp/height gradient tightens between the surface ridge extending south 
into the area and weak low pressure over the deep south and 
Carolina/southeast coast. Based on the 12z model runs...the timing 
of the arrival of precipitation is quite delayed compared to previous 
runs...with the best chance for rain starting in the south/SW in the 
afternoon...spreading northward into the Triangle area by or shortly 
after sunset. Will show highest probability of precipitation (categorical/likely) in the SW 
Piedmont and sandhills...tapering down to slight chance north/NE of the 
Triangle. Rainfall amounts should be no higher than 0.10-0.25" in 
the SW Piedmont/sandhills. Temperature forecast somewhat difficult on 
sun...depending primarily on the arrival of precipitation. Latest MOS 
guidance has come in warmer in almost all locations...ranging from 
the middle/upper 50s to the lower 60s...and that looks pretty 
reasonable based on the delayed onset of precipitation. -Vincent 


&& 


Short term /Sunday night through Monday night/... 
as of 315 am Saturday... 


As the upper system lifts northeast through the mountains Sunday 
night...low pressure development off the Carolinas occurs and 
becomes the dominate surface low. Good middle level isentropic lift in 
the middle levels should keep rain chances high into Sunday night. Most 
models damp out the parent low pressure trough as it moves northeast 
and keep the surface low weaker than the Canadian as it approaches 
Cape Hatteras. Rainfall perhaps two tenths inch northwest to four 
tenths southeast Sunday night. 


Winds weaken late Sunday night into Monday and heavier 
precipitation may miss US to the east Monday. Precipitation may 
linger longest in the northeast with the exiting surface low but 
rainfall amounts of one tenth inch or less east... and much less 
to the west. Of course if the low tracks closer to the coast... 
these amounts are underdone. Only a slight rain chance in the 
northeast with the exiting low Monday night. 


Overnight lows 45 to 50 Sunday night... and mostly middle 40s Tuesday 
night. Sunday high temperatures near 50 northwest to upper 50s 
southeast. If a cold air mass damming wedge sets up as expected 
Monday...a low overcast with light rain or drizzle possible. The 
current forecast Ten Degree diurnal rise to Monday afternoon would 
need to be dropped to three to six degrees if clouds are able to 
persist into early Monday evening. 


&& 


Long term /Tuesday through Friday/... 
as of 315 am Friday... 


Any light precipitation lingering into Monday night should be gone by 
Tuesday morning...as southwesterly middle level flow Ushers in drier 
air above the remaining cad airmass. The possibility lingering low 
clouds could complicate the temperature forecast on Tuesday as some 
moisture remains below a subsidence inversion beneath developing 
shortwave ridging aloft. Will stick with MOS values with highs 
64-70. 


The forecast gets quite a bit more complicated beyond Tuesday...with 
the handling of a shortwave expected to enter the Pacific northwest 
on Monday and emerge over the north Central Plains by Wednesday. The 
general trend is for the development of a high amplitude longwave 
trough by the end of the week...and while both the GFS and European model (ecmwf) 
resolve such a solution...they currently get the similar answer 
through somewhat different scenarios. The GFS sends the 
aforementioned shortwave across the upper Midwest and the lifts the 
system into Ontario by Thursday...pushing a dry cold front into 
central NC Wednesday night. This is followed by a second shortwave that 
dives south through the Midwest...but with no more to work 
with....points east of the mountains stay dry even as a surface wave 
is induced offshore Thursday morning. The European model (ecmwf) on the other hand 
is much slower with the first shortwave...allowing more moisture to 
be pulled north over a persistent surface ridge along the eastern 
Seaboard. Cyclogenesis along the coast subsequently occurs closer 
to the coast Thursday morning with less eastward progression of the 
surface front...and thus the European model (ecmwf) produces precipitation over the eastern 
third of NC. Given these continued model inconsistencies...feel the 
best course of action is to show below normal temperatures for the end of 
the week...continuing with higher slight chance probability of precipitation in the east 
until some sort of model convergence begins. 




&& 


Aviation /12z Saturday through Thursday/... 
as of 645 am Saturday... 


With models trending slower on the arrival of a storm system 
currently over the western Gulf Coast...VFR conditions will prevail 
through the valid taf forecast. Scattered to broken cirrus and eventually 
cirrostratus will drift across the region today and into 
tonight...all ahead of a storm system set to impact central NC from 
the southwest during the day Sunday. At the surface...high pressure 
to our north will settle over New England...extending a ridge of dry 
air down the East Coast....and providing a light NE wind. 


As the storm system approaches from the southwest Sunday morning 
ceilings will eventually lower to MVFR or IFR levels Sunday through 
Monday...with periods of rain possible at all taf sites. At least 
MVFR ceilings are expected to persist through late Monday...before a 
coastal low moves away from NC and pulls drier air into the area. 
Conditions are expected to remain VFR for the most through the 
balance of the week...although there is the potential for another 
weather system to effect mainly eastern terminals by Thursday. 


&& 


Rah watches/warnings/advisories... 
none. 


&& 


$$ 


Synopsis...Vincent 
near term...Vincent 
short term...99 
long term...Smith 
aviation...Smith 

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