480 
fxus62 krah 240248 
afdrah 


Area forecast discussion 
National Weather Service Raleigh NC 
945 PM EST Monday Nov 23 2009 


Synopsis... 
low pressure near Cape Hatteras will move slowly northeast away from 
our region through Tuesday. A light NE flow at the surface will keep 
conditions cool and moist. The temperatures will head back toward 
normal middle week as some sunshine returns. 
&& 


Near term /through tonight/... 
as of 945 PM Monday... 


The latest surface analysis indicated a weak surface low (1016 mb) 
very near Cape Hatteras. A strong surface high (1030 mb) remained 
centered over northern New England... but continued to extend SW 
through NC/SC and Georgia. The significant rain had already ended as of 
middle to late afternoon over our region as the surface low reached our 
latitude along the coast. However... residual cold air damming 
continued along and east of the mountains where low ceilings (at or 
below 900 feet)... patchy fog... and drizzle continued. The drizzle 
has been enough to measure a couple of hundredths in several places 
since 700 PM. 


The latest upper air data analysis indicated little support for any 
changes overnight. The Greensboro and Blacksburg radiosonde observations indicated 
saturation from the surface up to mountain top levels around 5k feet 
with the persistent NE flow at the surface with continued veering 
winds aloft. Radar indicated light 8-12 dbz returns associated with 
the patchy drizzle and very light rain. With little advection 
overnight... expect the boundary layer to remain saturated. Expect 
occasional drizzle and patchy fog. The temperatures should not fall 
but a few more degrees from the current readings in the upper 40s to 
middle 50s. 
&& 


Short term /Tuesday through Thursday/... 
as of 240 PM Monday... 


For tuesday: weak low pressure sits just off the northern Outer 
Banks Tuesday morning and is expected to drift northeast along the 
offshore baroclinic zone through Wednesday... leaving a very weak 
high pressure ridge at the surface with lingering weak troughs at 
925-850 mb. With the resultant weak and lightly diffluent surface 
winds and saturated low levels as noted on forecast soundings... 
patchy light drizzle and areas of fog should hold through Tuesday 
morning. Profound warm capping at 850-700 mb should help inhibit 
vertical mixing into the surface-based layer... and the NAM with its 
lighter winds through the lowest 1 km holds onto a nearly saturated 
layer through the day. But the gfs's slightly stronger winds above 
950 mb encourage horizontal mixing resulting in weaker stability 
within wedge air mass over central NC. Heating from above and some 
mixing-in of the dry air in the capped layer may start to slowly 
burn off the upper portion of the surface-based layer... however the 
more stubborn residual stable pool of the NAM is preferred based on 
its better terrain depiction and model agreement on lingering weak 
moisture advection in the northeast flow at 925 mb. With cloudy 
skies... still expect highs of 54-59. 


For Tuesday night/wednesday: very weak ridging at the surface and 
weak troughing at 925 mb both linger over NC overnight. With a 
steady feed of low level Atlantic air and no mechanisms via which 
this stable surface pool could disperse... expect a steady state 
Tuesday night with cloudy skies. The thick blanket of clouds should 
limit the diurnal drop. Lows 47-51. A middle level low near Chicago 
tracks northeast across Michigan as the next stronger vortex drops 
southeast toward Iowa... which keeps a broad cyclonic southwest middle 
level flow over the southeast states. The baroclinic zone holds just 
off the coast with the models indicating weak areas of low pressure 
tracking along it. The GFS pulls precipitation back across southeast NC 
whereas the NAM keeps the bulk of the lift and precipitation offshore. 
Given that the GFS features several spurious strong vorticity maxima 
that appear to be induced by earlier Gulf convection... favor the 
drier NAM and will keep it dry through Wednesday. Otherwise the low 
level col zone with weak/variable winds looks to persist through the 
morning... although the residual wedge air mass will likely be quite 
vulnerable by this point... with the higher terrain as the only 
remaining blockade restricting drainage. With the northward passage 
of a weak 850 mb trough through the area... low level winds finally 
swing around to southwest and west and scour out the low levels. 
Expect to finally see some sunshine break out in the afternoon... 
although with the amount of leftover low level moisture... would 
expect to see any holes in the sky cover to fill it with flat 
cumulus. Have leaned toward the cooler NAM statistical guidance and 
shaved a degree or two off highs... to 58-63... close to seasonal 
normals. 


For Wednesday night through Thursday night: the weather looks 
largely quiet for much of Thanksgiving day... but an intense storm 
system will bring increasing winds... a few sprinkles or showers... 
and a shot of colder air starting in the evening. The middle level 
vortex swings across northern Illinois and along the Michigan/Ohio border through 
Thursday. With minor timing differences the models are pretty close 
in sweeping the tail end of a sheared lobe of vorticity at 500 mb 
through western NC and Virginia by Thursday morning... although the 
projected moisture profiles would suggest few impacts on our 
sensible weather other than enhanced high/middle clouds. Surface winds 
remain light Wednesday night and we should see at least patchy fog 
through daybreak. The NAM/GFS/Canadian/ECMWF all indicate 850 mb 
cold air advection beginning late Thursday afternoon as the next 
strong middle level vorticity lobe and trough axis swing east then 
northeast toward the area. But the cooler air at the surface will 
remain held up on the west side of the mountains through sunset... 
resulting in steepening low level lapse rates Thursday afternoon. 
Expect partial clearing as any morning fog and stratus mix out... 
then clouds should increase again in the afternoon with the 
developing shallow instability... incoming DPVA... and rapidly 
increasing upper divergence in the left exit region of the 
strengthening (to over 130 kts) subtropical jet over GA/al... all 
producing rapid pressure falls at the surface with a low forming 
just north of the NC/Virginia border. The NAM/Canadian/ECMWF show the middle 
level trough taking on a negative tilt heading northeast through 
Virginia... and this more intense solution is preferred over the GFS which 
keeps a more wrapped system with a neutral to positive tilt. The 
moisture attending this trough is limited with no opportunity to 
advect any moisture in... however if the dynamic lift is strong 
enough it could condense sufficiently for a few showers across the 
northern counties. The potential also remains for a stratospheric 
intrusion in this type of dynamically vigorous pattern with 500 mb 
winds increasing to around 80 kts... however it appears from 
forecast soundings that the mixed layer might not deepen to more 
than 2 km so this is appears unlikely. Regardless though... the low 
level height falls and tightening pressure gradient will result in 
quickly increasing surface winds and gusts late Thursday afternoon 
through Thursday night. Expect sustained winds around 15 miles per hour with 
frequent gusts over 25 miles per hour... but with winds of 25-35 kts through 
the mixed layer Thursday night into Friday... would not be surprised 
to see occasional gusts approach 40 miles per hour. Lows Wednesday night 40-46. 
Highs Thursday of 60-66 still agree well with model thicknesses 
trends. Lows 36-43 Thursday night with decreasing clouds toward 
morning. -Gih 


&& 


Long term /Friday through Monday/... 
as of 255 PM Monday... 


For Friday/Friday night: expect continued brisk and gusty northwest 
winds within the mixed layer through Friday as the middle level vortex 
shifts east across the midatlantic coastal area toward New England. 
We should see just scattered clouds at most with a dry column and 
downslope flow. Low level thicknesses are nearly 25 M below normal 
with cold advection through the day. Will have highs 51-57. Lows in 
the Lower-Middle 30s Friday night with winds diminishing as surface 
high pressure starts to build in from the west-southwest. 


For Saturday through monday: quiet weather over the weekend with 
surface high pressure building into and over the area with ridging 
aloft over the southeast. Low level thicknesses recover back toward 
normal. Highs 54-59 Saturday and 57-63 Sunday with lows in the 
Lower-Middle 30s. The next storm system digs through the southern 
rockies late Sunday and shifts into the plains Monday. Gradually 
increasing overrunning should bring increasing clouds over NC 
Monday... but with this system still well to our west... will keep 
it dry for now. Highs again 57-63. -Gih 


&& 


Aviation /00z Tuesday through Friday/... 
as of 730 PM Monday... 


LIFR/IFR ceilings are expected to predominate through the taf period 
in association with a residual cold air damming wedge sitting over 
the Carolinas. Ceilings will vary little through most of the taf 
period...although visbys may vary quite a bit...from LIFR to 
MVFR/VFR through 12z Tuesday...and from IFR/MVFR to VFR during the day 
Tuesday. 


Residual cold air damming wedge and IFR/LIFR ceilings are expected 
to persist across the area Tuesday night into Wednesday morning. Latest model 
guidance indicates that The Wedge should begin to erode on Wednesday 
morning and early afternoon...and VFR conditions are expected by Wednesday 
aft/eve...continuing through the remainder of the week. -Vincent 


&& 


Rah watches/warnings/advisories... 
none. 


&& 


$$ 


Synopsis...Badgett 
near term...Badgett/krr 
short term...Hartfield 
long term...Hartfield 
aviation...Vincent 

National Weather Service Glossary of Abbreviations

Non-Expanded Version (with abbreviations)