528 
fxus62 krah 220228 cca 
afdrah 


Area forecast discussion 
National Weather Service Raleigh NC 
1028 PM EDT sun Mar 21 2010 


Synopsis... 
a strong cold front will approach from the southwest overnight... 
then sweep through our region Monday morning. The upper level storm 
system will track from the Tennessee Valley NE across Virginia Monday into 
early Tuesday... keeping the weather unsettled early this week. 


&& 


Near term /through Monday/... 
as of 1000 PM Sunday... 


Per the 00z upper air analysis...the strong middle/upper level closed 
low was located over north central Mississippi. Strong height were 
analyzed on the order of 180 to 200 meters over the lower MS valley 
and into the Tennessee Valley region. This upper low is forecast to 
move slowly east northeast into the Tennessee Valley by Monday 
morning. The associated surface cold front still looks on track to 
reach our western Piedmont zones between 06z-09z...rdu/Fay 
between 09z-12z...and then rwi/rzz between 12z-15z. A band of 
showers and thunderstorms currently moving across eastern Georgia is 
expected to increase in coverage as it is driven by an increasing 
moisture axis just ahead of the front associated with a 40-50kt low 
level jet. As this band of precipitation moves through central North 
Carolina overnight...any isolated thunderstorm that develops will 
have the potential to produce damaging wind gusts. However...with 
dewpoints generally only in the middle to upper 50s...instability 
continues to be lacking (cape values generally less than 300 j/kg) 
and therefore Storm Prediction Center continues to think only a marginal threat for 
severe weather exists overnight. The rain is expected to taper off 
from west to east through Monday morning as middle level dry moves into 
the region. -Krr 


The middle/upper low will have to be dealt with by middle to late 
afternoon as it begins a track from the southern Appalachians NE 
across northwest NC into southern Virginia by evening. It appears that there will 
be enough instability for a rapid increase in clouds by late morning 
or early afternoon. However... the surface moisture will become 
greatly limited as the middle level drying mixes down. The skies should 
become cloudy northwest and remain variably cloudy east Monday afternoon 
with a gusty SW wind. Isolated showers (possibly scattered in the 
west and north where there will be more available moisture)... close 
to the low track. SW winds of 15-25 miles per hour with gusts to 35 miles per hour are 
expected. 


&& 


Short term /Monday night through Wednesday/... 
as of 245 PM Sunday... 


Monday night and tuesday: the intense and cold upper low over the 
lower MS valley -- which deepened nearly 100 meters in 12 hours from 
00-12z/21st... and sports 500 mb temperatures around minus 30c -- will 
translate east northeastward across western NC/central Virginia Monday 
night. 850 mb-5 lapse rates steepen in excess of 6-7 c/km during the 
evening and overnight hours... owing to the arrival of the 
aforementioned cold pool aloft attending the upper low. This 
decreasing stability in conjunction with dynamic forcing for 
ascent and middle level deformation associated with the upper low 
warrant a continued slight to low chance of showers (most 
concentrated over the triad). Graupel (or less likely a wet flake 
or two if the precipitation persists toward daybreak in the higher 
elevations over the northwest piedmont) will be possible in the 
heavier showers. Lows generally in the middle-upper 30s northwest to Lower- 
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 from 60 to 65 degrees from north to south... provided the 
cloud cover is not more expansive or lingers longer than expected. 
It will also be quite breezy... with 15 to 20 miles per hour west winds gusting 
frequently into the 20s miles per hour during the afternoon. 


&& 


Long term /Wednesday night through Sunday/... 
as of 309 PM Sunday... 


Wednesday night through Thursday night: the nwp models continue to 
indicate a ridge-flattening northern stream trough aloft will 
propel a weak backdoor cold front through the region Wednesday 
night. Since the low level flow is projected to quickly veer to 
southerly or southeasterly over all but northeastern NC 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 west southwest flow 
aloft on Thursday night. Lows in the middle to upper 40s. 


Friday through saturday: while the weak backdoor front will likely 
have dissipated after retreating northward into Virginia by Thursday 
night... another much stronger cold front is expected to settle 
through the region on Friday... in advance of an approaching trough 
aloft from the west. Pending the timing of the front... some 
showers could develop along the front as it passes (if a late 
afternoon fropa). However... it appears that most of the 
precipitation will fall as stratiform...anafrontal rain later Friday 
afternoon and Friday night... owing to weak southwesterly flow atop 
the earlier southward-settling surface front (isentropic upglide). 
Northeasterly low level flow and possible broken stratocumulus 
behind the cold front will likely hold temperatures in the middle to 
upper 50s Saturday... while return flow ahead of the next frontal 
system should yield moderating temperatures into the 60s to near 70 
south on Sunday. 


&& 


Aviation /00z Monday through Friday/... 
as of 740 PM Sunday... 


A potent upper level low over the lower MS valley will move slowly 
northeast through the region and across the Carolinas through 
Tuesday. Conditions will deteriorate late this evening and into the 
overnight hours as strong warm and moist air advection overspreads 
the area from the south. Ceilings will lower to MVFR at western 
terminals between 00 to 06z with light rain showers occasionally 
producing some brief MVFR visibility restrictions as well. 
Meanwhile...central and eastern taf sites should hold at VFR. 


A convective line of showers...ahead of a strong cold front...is 
then expected to surge into our region from SW to NE between 
08z-13z Monday. Low level wind shear is expected between 06z-14z as 
the winds increase to 40-50kt at 2k feet during this time from the 
south-southeast. Isolated thunder is also possible especially at eastern 
terminals where instability will be slightly marginal to support 
stronger updrafts. 


Behind the cold front... rapid drying will ensue with VFR 
cigs/vsbys. Winds will become SW at 12-22kt with gusts to 30kt late 
morning into the afternoon. Only isolated showers are expected but 
VFR ceilings are likely along with the gusty winds. Another period of 
MVFR ceilings are possible Monday night and into Tuesday morning with 
the passage of the upper low. A return to VFR ceilings/visibilities is expected 
Tuesday into late week. 


&& 


Rah watches/warnings/advisories... 
none. 


&& 


$$ 


Synopsis...Badgett 
near term...krr/Badgett 
short term...mws 
long term...mws 
aviation...cbl/Badgett 

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