Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Raleigh NC
315 PM EDT Wednesday Jul 30 2014
Synopsis...weak high pressure will remain over the area today with
mostly dry conditions before moisture associated with an upper level
trough and retrograding coastal front causes precipitation to become
more likely for the end of the week into the weekend.
Near term /through tonight/...
as of 245 PM Wednesday...
Showers starting to break out mainly along the Highway 1 corridor
highlighted on the visible satellite imagery by a more enhanced
cumulus field. This was captured well on high resolution
models...particularly the latest run of the hi-res WRF nmm. This
solutions continues showers through the evening but does not enhance
activity much. Therefore will keep slight chance of showers and
introduce isolated thunder in the forecast...as although moisture
appears very shallow at this time...Storm Prediction Center surface analysis suggests
500 j/kg of mixed layer cape over central NC and a small area of 25-
30 knots effective shear in the kfay area. Cloud cover has limited high
temperatures...especially in the north and west and will likely top
out in the upper 70s northwest to low to middle 80s in the extreme south.
Expect to have cloud cover and possibly a lingering shower run into
the overnight hours which should help limit fog/low stratus but a
brief period of both will be possible at krdu...kfay...and krwi just
before sunrise. The increase in cloud cover should help keep low
temperatures up a couple degrees from last nights lows...mainly low
to middle 60s across the area from northwest to southeast.
Short term /Thursday and Thursday night/...
as of 245 PM Wednesday...
The upper level trough will remain draped across the eastern Continental U.S.
On Thursday. At the low levels southerly flow will begin to return
to the area...increasing moisture in the southwestern County Warning Area via return
flow. This process will take a while to get going however and most
of the morning/early afternoon should remain dry with increasing
cloud cover from the southwest during the day. After 18z...the
chances for showers and potentially a thunderstorm will increase
with most model simulations producing quantitative precipitation forecast by 00z Friday as
precipitable water values begin to climb towards two inches.
Instability will be very limited Thursday afternoon and evening
although about 40 kts of bulk shear will be available. Any
showers/storms that do commence Thursday afternoon will have a
chance of continuing through the overnight hours as some elevated
instability may be present in the form of a vorticity maximum moving through
on the eastern side of the upper trough from southwest to northeast.
With increase clouds from the southwest...maximum temperatures may be highest
in the northeast on Thursday with temperatures in the middle to upper 80s and
only lower 80s in the west and southwest. Overnight lows Thursday
night will moderate into the upper 60s.
Long term /Friday through Wednesday/...
as of 315 PM Wednesday...
The overall pattern during the long term period features an eastern
Continental U.S. Trough and a western U.S. Ridge which will support near or
cooler than average temperatures and near or greater than normal
threat of precipitation.
The broad eastern U.S. Trough sharpens across the lower Mississippi
Valley on Friday and a strengthening inverted trough/coastal front
develops. An increasingly moist environment combined with isentropic
lift will promote a good chance of showers and thunderstorms Friday
afternoon and evening that will linger but become more scattered
The upper trough sharpens further over the weekend as the inverted
trough moves inland and deep moisture overspreads the region.
Precipitable water values climb to around or just over 2 inches
which is 2 Standard deviations above climatology during the weekend.
The axis of greatest moisture extending south-northeast across
central NC. Nwp guidance advertise multiple disturbances in the
sharpening upper trough that will likely enhance precipitation
coverage and intensity although it is difficult to Pin Point the
timing and location at this point. For now will include likely probability of precipitation
for Saturday and very high chance probability of precipitation for Sunday with the greatest
coverage during the afternoon and evening hours.
Both the GFS and the European model (ecmwf) suggest a portion of the trough will
break off and move across the northeast on Monday and offshore
Tuesday while a portion of the tough lingers across the lower
Mississippi Valley. This should result in a decrease in convective
coverage and more typical chances of mainly afternoon and evening
thunderstorms. Another disturbance drops into he trough on Wednesday
bring a cold front with it that could once again enhance
precipitation chances for the middle week.
High temperatures will generally range in the upper 70s to lower 80s on
Friday through Sunday an then moderate slightly into the lower to
middle 80s by middle week. Morning lows will range in the middle 60s to near
70 for most of the period.
Aviation /18z Wednesday through Monday/...
as of 140 PM Wednesday...
24 hour taf period: mostly VFR conditions expected through the end
of the taf period. Scattered to broken cloud cover at the 3-7 kft
range may lead to some shallow shower activity this afternoon.
Otherwise variable winds at less than 5kts are expected for the rest
of the day. Overnight tonight there remains a small chance of some
sub VFR ceilings and visibilities...primarily at central and eastern
sites between 10z and 12z. There will be an increasing chance for
showers and thunderstorms tomorrow afternoon...but largely after the
end of the taf period. Otherwise broken ceilings at 5 kft should
prevail through 18z with winds less than 5 kts generally out of the
Long term: chances for showers and thunderstorms will increase
Thursday evening through the weekend and into early next week as a
coastal trough retrogrades westward into the coastal plain.