Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service Raleigh NC
710 PM EST sun Dec 21 2014

Synopsis...high pressure to the north will set up a cold air
damming event with precipiation chances increasing tonight and into
Monday as a surface low moves up the southeast coast.


Near term /through Monday night/...
as of 220 PM Sunday...

High pressure over the Middle Atlantic States is slowly shifting
eastward this afternoon and into a favorable position for a hybrid
cold air damming event to begin. This is keeping moisture penned up
south of the area today but a southern stream shortwave will begin
to create some troughing along the coast overnight tonight into
Monday which will help to sharpen up The Wedge front across central
NC and set the stage for a wet couple of days.

For the rest of today expect less cloud cover east of Highway one
with lingering MVFR ceilings in the west. This has been trying to
burn off over the past several hours but some places are still under
Grey skies. This is creating a lot of differential heating across
the County Warning Area and any locales in the clearing have temperatures rising
above forecasted highs and those under the cloud cover are still
below forecasted highs. The end result will be plus or minus 50
degrees depending on location. Winds are maintaining a northeasterly
component and will continue to do so with speed around 5 kts.

Tonight...moisture will begin to increase from the south aided by
isentropic lift increasing over Top of the Wedge that will begin to
build in this evening...and the right entrance region of an upper
jet that is prognosticated to begin to move over central NC during the late
night hours. High resolution models have precipitation beginning in
the southern counties after 00z and spreading northward through the
County Warning Area by about 4z or so. The onset of precipiation will also mark the
beginning of MVFR ceilings. Low temperatures will range from the middle
30s north to low 40s south.

Monday and Monday night: rain is expected to persist through the day
on Monday as the surface low moves northward just off the NC coast.
Low ceilings at IFR levels are expected throughout the day. While
rain is expected to be continuous...the quantitative precipitation forecast amounts with this system
are fairly low. Only expect a total of a quarter to a half of an
inch of rain. With The Wedge locked in to the west and the coastal
trough to the east...there should be a fairy decent gradient from
northwest to southeast in the high temperature spread on Monday.
Highs in the northwest Piedmont will struggle to get to 40 degrees
whereas highs closer to 50 will be common in the southeast. For the
most part precipiation is expected to end after 00z Tuesday but
continued moisture in the area will lend itself to continuing
drizzle through the night on Monday. This wet and cool period will
also lend itself to periods of patchy fog with the most likely times
before sunrise on Tuesday. Lows in the middle 30s to lower 40s.


Short term /Tuesday through Wednesday night/...
as of 300 PM Sunday...

middle-level dry air and the absence of discernible
forcing...aside from weak southerly upglide...downstream of the
deepening trough will keep measurable rain chances low across
central NC during much of the day on Tuesday. Despite a lack of
precipitation...shroud of low clouds will help to keep the lingering cold
air wedge largely in place across central NC. Measurable rain
chances will return by the late afternoon/evening with the
resurgence of warm moist advection and low-level convergence along
the northward retreating wedge front. Temperatures Tuesday during
the day will remain the 40s throughout much of the day...the
exception will be across the far southeast zones where nearby wedge/warm
front could allow temperatures to warm into the lower 50s late in
the day.

Tuesday night through Wednesday night:

Soggy...breezy...and possibly stormy conditions...

Ahead of the amplifying upstream longwave trough and attendant surface
cold front...deep Gulf moisture feed will transport anomalously high
precipitable waters of 1.5-1.7" into the region...which would rank in the 99th
percentile(highest observed precipitable water during the month of Dec at kgso
based on the radiosonde database which date backs to 1948 is 1.59"-
12/21/1991). Strengthening right entrance jet region coupled with
this strong moisture flux convergence will result in categorical
probability of precipitation and a widespread soaking rain of 1-2"...heavy at times Tuesday
night and into Christmas evening morning. Once the warm front lifts
northward into the Virginia...there will likely be a lull in precipitation
until the arrival of the cold front and attendant band of showers
and possibly embedded thunder through the area between 00-06z
Thursday...which is a bit slower than yesterdays timing.

Severe potential: evaluating severe potential in a Low Cape-high
shear environment is always difficult...even at closer time scales
of 6-12 hours...much less at the current time scale of 3 to 4 days.
The lull in precipitation during much of the daytime hours...may allow for
some modest heating...especially from the US 1 corridor
eastward...which could lead to better destabilization than model
currently show(100-200j/kg mucape). Weak lapse rates aloft of 5.0-
5.5 c/km along with the main dynamics passing over the Great
Lakes...will limit widespread severe potential...but given
impressive deep layer shear cannot rule out isolated severe storm at
this time.

Temperatures very challenging with high bust potential...dependent
on evolution/timing/location of the retreating wedge front into the
climatologically favored area across the far northwest Piedmont Tuesday
night and into the day on Wednesday. Expect temperatures to steadily
warm after midnight Tuesday night...with the exception being across
the northwest Piedmont...where temperatures could hold nearly steady. Lows ranging
from lower 40s northwest to middle 50s southeast. Warm sector should overspread much
of the area by Wednesday afternoon...warming into the 60s across the
Piedmont...and possibly lower 70s across the sandhills/southern
coastal plain. Rapid clearing and cool air advection will commence
Christmas evening evening/night with lows Christmas morning in the upper
30s northwest to Lower/Middle 40s southeast.


Long term /Thursday through Sunday/...
as of 255 PM Sunday...

Deepening surface low lifting northeastward into southeast Canada will result in
breezy conditions on Christmas day. Otherwise dry with seasonably
mild temperatures continuing into Friday as high pressure builds in
from the northern Gulf.

Model spread begins to increase dramatically late Friday through the
weekend...with the ec much deeper with the next middle/upper level
trough...resulting in yet another wet and unsettled period next
weekend. Meanwhile...the less amplified GFS indicates quasi-zonal
flow across southeast US with dry conditions through the weekend.
Will indicate slight chance probability of precipitation until better agreement is reached.


Aviation /00z Monday through Friday/...
as of 710 PM Sunday...

There is high confidence that aviation conditions will deteriorate
tonight as a low pressure system begins to develop off the southeast
coast. While a few patches of light rain will be possible in the
next few hours... expect the light rain currently over SC to expand
and spread into NC after 06z...with ceilings eventually lowering to
MVFR and IFR...first at kfay/kgso/kint by 06-08z and then krdu/krwi
by 09z. Once IFR ceilings develop...forecast sounding show very
little improvement...if any...during the day on Monday. The rain
will transition to just drizzle as the day progresses...but poor
aviation conditions will persist through the end of the taf period.

Long term: poor conditions with low ceilings/visibilities and periods of
rain will continue through Tuesday and into Wednesday. A cold front
will cross the area on Wednesday...bringing strong winds and a
chance of a few thunderstorms. Conditions should return to VFR for
Christmas day with some afternoon gusty winds possible.


Rah watches/warnings/advisories...



near term...Ellis
short term...cbl
long term...cbl

National Weather Service Glossary of Abbreviations