141 
fxus62 krah 200716 
afdrah 


Area forecast discussion 
National Weather Service Raleigh NC 
315 am 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 235 am Saturday... 


Middle/upper level short wave ridge and attendant surface ridge will move east 
this morning. Return southerly flow on the back side of surface ridge will 
gradually increase the low level moisture across the area as well as 
advect slightly warmer air. Models were 5-6m too cool with 
thicknesses Friday afternoon. Model forecasts suggest thicknesses in the 
middle 1370s. These values coupled with plenty of sun and steady S-SW 
wind should result in maximum temperatures today 3-5 degrees warmer than those 
observed Friday (max temperatures 74-79). 


Tonight...SW flow aloft will begin to advect middle/high level clouds 
especially after 06z in the west and south. This should set up a 
temperature gradient between the warmer SW and the cooler NE. Min temperatures 
will ultimately depend upon existence/persistence of a near surface 
southerly flow. If winds decouple...potential for min temperatures in the NE to 
be a few degrees cooler. (Min temperatures middle/upper 40s NE - 50 sw). 


&& 


Short term /Sunday through Tuesday/... 
as of 300 am Saturday... 


Deep upper level low over the arklatex early Sunday will lift east-NE 
as strong upper jet rounds base of the trough. Increasing SW flow 
aloft will advect moisture ahead of this system so expect clouds to 
thicken and lower from the SW throughout the day. However...still 
expect enough sun early in the day( and likely into the early 
afternoon over the ne) in conjunction with a steady southerly surface wind 
to boost temperatures into the low/middle 70s with warmer temperatures east of 
Highway 1. Low level flow will increase after 18z with model rn 
cross sections now suggesting that there will be enough moisture 
available through the column to support a few showers late in the 
day over the southern and western Piedmont. Previous forecast had 
good handle on this but made slight adjustment to delay precipitation 
chances a few hours in the northwest. 


Sunday night...increasing low level jet in response to the approach 
of the deep middle/upper level low will reach speeds around 40kts later 
Sunday night. Meanwhile near surface flow will back to a southeast 
direction. This will result in a high shear environment late Sunday 
night into early Monday with 0-1km storm relative helicity 300-400 
m2/s2 (gfs stronger than NAM-wrf). While shear values impressive... 
low level air mass prognosticated to be quite stable with surface base cape 
remaining below 300 j/kg. Thus...while could see organized bands of 
heavy showers/isolated thunder Sunday night into early Monday...lack 
of sufficient low level instability will limit severe threat. MOS 
guidance continues trend of increasing probability of precipitation Sunday night. Will 
adjust probability of precipitation up a few percentages and paint low end categorical probability of precipitation 
across much of the region after 04z Monday. 


Monday...middle level dry slot ahead of mature cyclone over the Tennessee 
Valley Monday morning will advance northeastward across the region. This will 
diminish shower coverage Monday morning with potential for some 
partial clearing by late morning over the southern and western 
Piedmont. Low level west-SW flow will limit low level cold air advection behind the 
front initially though thickness fall 25-30m through the day Monday. 
In these type of events...warmer guidance is usually more correct so 
will maximum temperatures Monday closer to the warmer NAM MOS. (Near 70 NE to 
middle 60s far W-sw). 


Monday night through tuesday: 
difficult forecast for this period... with regard to probability of precipitation and cloud 
cover. The closed upper low is forecast to track across the region 
during this period... possibly producing some showers across the 
area. The latest GFS has trended a bit more northward with the track 
of the low... tracking it across our state in a southwest to 
northeast fashion... with the best precipitation chance across our north. 
The 12z ec tracks the middle level low across South Carolina... and has 
the best chances for precipitation across the south. Thus... for now will 
keep only slight chance probability of precipitation across the entire area. However... once 
the track of the middle level low becomes more clear... at least chance 
probability of precipitation will likely be need for a portion of the County Warning Area. Also... we will 
need to keep an eye on thickness values and thermal profiles on 
Tuesday morning... given the cold pool aloft. The GFS (ec is too 
warm) has critical thickness values dropping into the rain/snow mix 
part of the nomogram. Model soundings off of the GFS show the 
potential for possibly some snow flakes mixing in with higher 
intensity rain showers... as the middle levels are cold enough to 
support snow... while the 1000-850 mb thickness values are marginal. 
However... with overnight low expected to range from the upper 30s 
to lower 40s... do not expect any impact. 


For Tuesday any lingering chances for showers will end by noon or 
shortly after as the upper low moves off to the east-northeast and a 
secondary cold front moves through the area. Skies will likely 
remain partly to mostly cloudy during the day... with locals across 
the southwest seeing more sun than areas across the northeast. 
Afternoon low level thickness values will struggle to reach 1340 
meters on Tuesday... some 10 to 15 meters below normal. Thus... even 
with an increasing downslope flow Tuesday afternoon... expect temperatures 
to remain below normal for this time of year. Expect high temperatures on 
Tuesday to range from the upper 50s across the northwest to lower 
60s elsewhere. 




&& 


Long term /Tuesday night through Friday/... 
as of 250 am Saturday... 


Medium range portion of the forecast looks to start off dry with 
afternoon high temperatures returning to back above normal on Wednesday as 
middle level ridging builds across the area... while surface high 
pressure settles across the southeastern U.S. This will yield dry 
and mostly clear/sunny conditions for Wednesday. Morning low are 
expected to be in the upper 30 to lower 40s. With low level 
thickness values increasing into the 1360s Wednesday afternoon... 
expect highs will top out in the lower 70s across the area. 


Medium range models being to diverge on Wednesday night... as the 
latest GFS (00z/20th) now has a northern stream short wave trough over 
southern Canada and the upper Midwest and Great Lakes tracking a bit 
further south and is stronger. While the main energy with this 
system and surface low are expected to remain well to the north of 
the central NC as the system track into portions of the northeast on 
Wednesday night... high pressure building across the Ohio Valley and 
Great Lakes region has just enough to push to push a backdoor cold 
front into our area Thursday morning. The GFS does not generate any 
quantitative precipitation forecast with the system... but does increase cloud cover and plunges low 
level thickness near to slight below normal for Thursday. However... 
given the previous runs of the GFS showed this feature remaining to 
the north of the area and the ec continues to keep the cold front to 
the northwest of the area... will continue to advertise above 
normals highs and lows on Thursday. Expect lows Thursday morning to 
be in the lower to middle 40s... with afternoon highs in the lower to 
possibly middle 70s. 


Next system in the southern stream is expected to move into the area 
on Friday... ushering in unsettle weather back into the forecast. 
For now... will keep probability of precipitation in the chance category... and highs temperatures 
near to slightly above normal (64 north to around 70 southeast). 
Lows Friday morning will likely be above normal as southerly flow 
ahead of the system should usher in increasing moisture and cloud 
cover. Thus... will go with lows in the middle to upper 40s. 


&& 


Aviation /06z Saturday through Wednesday/... 
as of 1250 am Saturday... 


VFR conditions to prevail through Sunday afternoon. An area of high 
pressure overhead at 05z Saturday will drift east through this 
afternoon. Return flow on the back side of the high will result in a 
steady SW wind at 7-11 knots by late morning persisting through the 
afternoon. Aside from a few passing cirrus after 15z...expect clear 
skies. 


Expect fair skies Saturday night with middle and high level clouds 
thickening Sunday as a deep upper level low pressure system 
approaches from the W-SW. Moisture may be deep enough by late Sunday 
to trigger/support a few showers in the western and southern 
Piedmont prior to sunset. Increasing low level jet may cause 
marginal low level wind shear conditions late Sunday night into 
early Monday. Otherwise showers will become more numerous after 
sunset Sunday with the greatest threat/coverage of showers and 
isolated thunder between 04z-12z Monday as a cold front approaches 
from the west-SW . Thus expect ceilings to lower into the MVFR range 
late Sunday night...persisting into Monday morning. 


Conditions will be slow to improve Monday into Monday night as the 
upper level low tracks across the region. Expect conditions to 
slowly improve Tuesday with a return to VFR conditions Tuesday 
afternoon through Wednesday. In wake of the departing low 
Tuesday...surface winds out of the west-northwest may be breezy with frequent 
gusts between 20-25kts. 


&& 


Rah watches/warnings/advisories... 
none. 


&& 


$$ 
Synopsis...wss/Badgett 
near term...wss 
short term...wss/bsd 
long term...bsd 
aviation...wss 

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