Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Raleigh NC
1046 am EDT Wednesday Aug 27 2014
Synopsis...high pressure will extend southward into the region
today. A back door cold front will settle south into central NC Thursday
and Thursday night.
Near term /through Thursday/...
as of 1045 am Wednesday...
The early morning stratus that developed across the east has burned
off quickly over the last couple of hours and has left mostly sunny
skies in its wake. Otherwise...Hurricane Cristobal will continue to
accelerate off to the northeast (but well offshore)...leaving
central NC on the dry subsident side of the storm. This along with
the fact that precipitable water values will remain below 1 inch...another dry and
pleasant day is expected. With maximum low level thickness values
prognosticated to be 15-20m higher today...high temperatures should be 3-5 degrees
warmer today than yesterday. Therefore...generally middle to upper 80s
are expected. -Krd
Surface high pressure and the core of the lower precipitable waters are forecast to
drift to the south of the area overnight as a backdoor cold front
begins to approach the area from the north... associated with the
short wave trough that is forecast to absorb Hurricane Cristobal and sweep
him quickly off to the northeast (away from any coastal areas in the
U.S.). lows tonight will again be comfortable for late August in
central NC... below normal in the lower to middle 60s... maybe even a
few locations touching the upper 50s.
Thursday: the previously mentioned backdoor cold front is expected
to drop southward into central NC Thursday afternoon. Moisture will
increase across the area in advance of this feature. However... with
low level westerly to west-southwesterly flow in advance of the
system we should see dewpoints in the lower to maybe middle 60s (as the
front approaches... pooling along it). This should lead to only
isolated convection developing in the afternoon across northern
portions of the area. However... we should warm up nicely on
Thursday... with the west-southwest to w'erly flow. Thus... expect high temperatures
to be in the lower to middle 90s. With respect to a severe threat... severe weather
potential looks to be low... given the rather weak instability...
thanks to the relatively dry low levels. The NAM appears to be too
moist and consequently generates too much instability as noted in
the previous afd for the pattern. In addition... deep layer shear is
forecast to remain weak... ~20kts or so. Thus... we are not
outlooked in the Storm Prediction Center day 2 convective outlook in anyway. If we get a
decent storm develop though it would have a damaging wind threat
thanks to the dry deep sub-cloud layer.
Short term /Thursday night through Friday night/...
as of 3 am Wednesday...
Backdoor front projected to stall in vicinity of the northern
Piedmont-northern coastal plain Thursday night then slowly dissolve
on Friday. Enough moisture pooling along this feature may aid to
trigger an isolated shower overnight Thursday night to the North-East
of rdu...with an isolated shower or T-storm possible over the same
region Friday afternoon. Building middle-upper level heights and
associated middle level subsidence will inhibit convective development
elsewhere. Afternoon thicknesses still averaging 10m above
normal...supportive of maximum temperatures about 3-4 degrees above normal.
This yields high temperatures Friday in the 90-95 degree range. Min temperatures
will be in the middle-upper 60s Thursday night and upper 60s-around 70
Long term /Saturday through Tuesday/...
as of 3 am Wednesday...
Saturday...presence of middle-upper level ridge over the forecast area
should inhibit convective development due to subsidence inversion
aloft. Still expect an extensive cumulus field as low level moisture
will be in the increase...with some growth to the cumulus until it hits
the inversion. Model guidance places the 700mb anti-cyclone directly
overhead. This favorable placement suggest maximum temperatures back in the 90-
95 degree range. Saturday night should see partly cloudy skies with
patches of fog developing late. Min temperatures in the upper 60s-lower
Middle/upper level ridge will gradually drift east and weaken with
time. Increasing SW flow aloft will increase moisture in the upper
levels of the atmosphere. While model guidance suggest that the bulk
of best upper level support for convection will remain west-northwest of
our region...weak perturbations in the flow aloft interacting with
available moisture and weak-moderate low level instability will
support the development of isolated to scattered convection Sunday
afternoon-evening...and scattered convection Monday afternoon-
evening. May see convection become a little more
concentrated/organized Monday and more so Tuesday as middle level
trough approaches and crosses the region. (Gfs as much as 12=18
hours faster compared to the European model (ecmwf). Per initialization tonight off
teh Pacific northwest...slower European model (ecmwf) preferred at this time.) Rainfall
associated with the heavier showers and storms may be locally heavy
as air mass will become tropical-like. Precipitable water values
predicted to return to near 2 inches by late Sunday or Monday. Our
stretch of above normal temperatures expected to persist ito early next
week with maximum temperatures in the upper 80s northwest-lower 90s southeast.
Aviation /11z Wednesday through Sunday/...
as of 635 am Wednesday...
24 hour taf period: Canadian high pressure overhead...between a cold
front approaching from the Midwest and Hurricane Cristobal
offshore...will result in mainly VFR conditions through the taf
period. However... an area of low stratus and fog has
developed/moved into eastern portions of the area this morning...
creating LIFR conditions at krwi. This area of low clouds and fog is
expanding west and southward this morning. However... confidence
that it will reach kfay and/or krdu before it begins to lift is low
to moderate. Think we will still see a brief period of stratocumulus
this morning from the moisture in the low level NE to north-northeast low level
flow combined with daytime heating at krdu and kfay though. Expect
this cloud cover will lift and scatter by middle to late morning...
with VFR expected by afternoon everywhere at the latest.
Otherwise... a generally light northerly surface wind is expected today
(5-10 kts). However... there will likely be a short period of
somewhat stronger winds (8-12 kts with gusts into the middle-upper
teens kts... mainly at kfay/krwi) at eastern sites middle-morning to
midday today...where the mslp pressure gradient will be residually
tight early in the day...between the overhead surface high and the
The low level flow is expected to become more northwest to westerly
overnight. Thus... do not expect a repeat performance of IFR/LIFR
morning fog and/or low stratus on Thursday morning. However... can
not completely rule out some MVFR visbys at krwi in the pre-dawn
hours of Thursday... though probabilities are way too low to include
in the taf at this time.
Outlook: the arrival of the cold front approaching from the upper
Midwest will result in a small chance of an afternoon shower or
storm Thursday and Friday...as well as a chance of MVFR ceilings in brief NE
flow Thursday night from krwi and especially points northeastward... into
NE NC and Tidewater Virginia. The probability of patchy fog will also
increase by the weekend...as the low level flow assumes a more
southerly (moist) component.