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afdrah 


Area forecast discussion 
National Weather Service Raleigh NC 
300 PM EDT Sat Mar 20 2010 


Synopsis... 
high pressure will linger over our region through Sunday bringing 
dry warm days and cool nights. A storm system will approach from the 
west Sunday night and affect the area Monday and Monday night. 


&& 


Near term /through tonight/... 
as of 225 PM... 


Increasing high cloudiness tonight with light winds. Periods of 
calm... longer and more likely after midnight. With morning 
thicknesses near 1358 meters this would equate to upper 40s to near 
50 for low temperatures. 


&& 


Short term /Sunday through Tuesday/... 
as of 225 PM... 


Increasing south southeast 850 millibar flow Sunday as clouds 
begin to lower and thicken. Weakest flow is over the east and 
divergent which should keep the east dry Sunday. Stronger 
winds...weakly convergent cyclonic flow and increased moisture 
over that in the east may make for late afternoon showers Sunday 
in the west. However the moisture and instability in the west is 
still only marginally favorable for precipitation before dark so 
rain chances for that area will continue low. 


Lack of instability continues to hinder thunderstorm potential 
Sunday night. Middle level temperature lapse rates are weak as are 
surface based lifted indices. Only helicity rising to in excess of 
300 as southwest middle level winds near 50 knots top southeast 
surface inflow is cause for concern. Am not sold on there being 
any thunder at all for this event... but should it develop... it 
is likely to be well organized and will have to be monitored. 


850 millibar cold front still forecast to sweep through central 
North Carolina Monday from the southwest. This will clear rain 
chances from the southwest near sunrise Monday and toward evening in 
Roanoke Rapids to the northeast. 


Widespread rainfall amounts for the event of around one half inch 
with locally heavier amounts. 


Afternoon high temperatures in the upper 60s northwest to middle 70s 
southeast Sunday... then middle to upper 60s Monday as low level 
thicknesses fall during the day in cold advection. Low temperatures 
Sunday night ahead of the front in the middle and upper 50s. -99 




Monday night and tuesday: in the wake of the earlier (monday) warm 
conveyor belt precipitation and trailing middle level dry slot... the 
upper low taking shape over the central US this afternoon will 
translate east northeastward across NC/Virginia Monday night. 850 mb-5 lapse 
rates steepen in excess of 6-6.5 c/km overnight... owing to the 
arrival of a cold pool aloft characterized by 500 mb temperatures in the minus 
upper 20s attending the upper low. This decreasing stability in 
conjunction with dynamic forcing for ascent ahead of the upper 
low... and an associated low level trough/convergence axis... 
warrant a slight to low chance of showers overnight. Forecast 
partial thicknesses fall in the indeterminate (too few cases) 
category... which is typical of steep lapse rate environments with 
above freezing low levels. Will accordingly keep the wording all 
liquid... though suspect there could be a few wet flakes in the 
higher elevations over the northwest Piedmont if a high enough 
precipitation rate occurs and is maintained - an unlikely occurrence 
at this time. Lows generally in the upper 30s northwest to lower to middle 
40s southeast. 


The upper low is forecast to remain progressive and move far enough 
northeast of central NC to preclude any instability shower 
development Tuesday afternoon. Scattered to broken stratocumulus in 
cyclonic flow aloft will persist... however... especially through 
early afternoon and over the northeastern third of the County Warning Area (in 
closer proximity to the departing upper low and associated middle level 
moist axis). Dry adiabatic mixing of prognosticated 850 mb temperature suggest 
highs only in the upper 50s to lower 60s... provided the cloud cover 
is not more expansive or lingers longer than expected. -Mws 


&& 


Long term /Tuesday night through Saturday/... 
as of 300 PM Saturday... 


Tuesday night and wednesday: dry... with moderating temperatures... 
courtesy of mild high pressure centered over the Gulf Coast 
states... beneath building heights aloft. Lows within a few degrees 
of 40... and highs generally in the lower 70s. 


Wednesday night through Thursday night: the nwp models continue to 
indicate a ridge-flattening northern stream trough aloft will propel 
a frontolytic backdoor cold front through the region Wednesday 
night. Since the low level flow is projected to quickly veer to 
southerly by Thursday afternoon... suspect temperatures Thursday 
will be within a couple of degrees of those of Wednesday. Lows 
should actually be milder than previous nights owing to low level 
mixing and minimal cold air advection in northerly flow behind the front Wednesday 
night... then as a result of thickening middle-high clouds in southwest 
flow aloft on Thursday night. Lows in the middle to upper 40s. 


The next chance of rain will come Friday and especially Friday 
night... in association with a trough aloft and attendant frontal 
system expected to cross the region. Turning cooler in the wake of 
this system on Saturday. 


&& 


Aviation /19z Saturday through Thursday/... 
as of 135 PM... 


VFR conditions to prevail through Sunday afternoon. South to 
southwest winds 10 miles per hour or less decreasing overnight. South winds 
near 10 miles per hour by middle morning Sunday with gusts to 20 miles per hour by early 
afternoon. No ceilings expected below 15000 feet. 


Lowering and thickening clouds Sunday afternoon with a few showers 
possible near and west of Highway 220 before sunset. Showers and 
perhaps a thunderstorm will spread across central North Carolina 
Sunday night. Rain ending near sunrise in the southwest and toward 
Monday evening in the northeast. Isolated showers possible roughly 
midnight Monday night to noon Tuesday... then dry with VFR 
conditions. MVFR to occasional IFR ceilings and visibilities in 
showers. Gusty winds of 15 to 25 miles per hour late Sunday night into 
Monday. 


&& 


Rah watches/warnings/advisories... 
none. 


&& 


$$ 
Synopsis... 
near term...99 
short term...99/mws 
long term...mws 
aviation...99 

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