Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Raleigh NC
840 PM EST Friday Dec 26 2014
Synopsis...high pressure will remain over the Carolinas through
Saturday... then shift offshore Saturday night. A cold front will
approach from the northwest on Sunday... and gradually move
southward through the region on Monday.
Near term /through Saturday/...
as of 840 PM Friday...
Another quiet night with both surface high pressure and middle level
ridging overhead and deep dry and stable air from the surface up
through 500 mb. A slug of higher level moisture tracking around the
periphery of the ridge... now moving over TN/KY/WV/VA... will affect
far northern and northwest County Warning Area with a period high thin clouds... however this is
unlikely to have much if any impact on temperatures. Winds have gone calm
in most areas... facilitating a good temperature drop this evening.
Dewpoint depressions are already down to just a few degrees
especially in the east... where dewpoints are generally higher than
in the west... and the latest hrrr depicts areas of fog forming
after 09z along the I-95 corridor. Have added a mention of fog in
the coastal plain late tonight into Sat morning. Otherwise... expect
mostly clear skies. Lows 29-36... lowest in outlying areas. -Gih
The high will begin to slip offshore on Saturday ahead of our next
system. The airmass will undergo further modification as return flow
sets up over the region. Highs should rise into the upper 50s to
near 60 degrees across the northwest (where middle/high clouds will be
first to arrive) and into the low/middle 60s further southeast. -Krd
Short term /Saturday night through Sunday night/...
as of 305 PM Friday...
Saturday night...SW flow aloft will usher in a series of
disturbances from the northern Gulf into the southern
Appalachians/mid-Atlantic region. Lead disturbance projected to
advance toward our region late Saturday night. Middle level flow will
gradually veer in response to this feature...aiding in moisture
transport into the region. Surface ridge is expected to remain aligned
west-southwest-east-northeast across central NC...maintaining a dry air presence. Low
level isentropic lift anemic during this period so appears lift
generated aloft from warm air advection. Expect dry conditions through Saturday
evening with patches of light rain showing up on the radar after
midnight in the west. Initial patches of rain will go to moisten the
sub cloud layer. Thus...best shot of measurable rain will likely be
confined to the far western-southern Piedmont.
Sunday...atmosphere expected to remain adequately moist to maintain
considerable cloudiness. Lead perturbation expected to cross just
west-northwest of our region in the morning. Bulk of precipitation with this
feature should occur in vicinity of the Piedmont counties with rain
coverage decreasing to the southeast. Will trend probability of precipitation from likely northwest third
to chance far southeast. By afternoon...expect nothing more than spotty
precipitation at this time with chance probability of precipitation areawide. Maximum temperatures Sunday
dependent on areal extent of rain or showers. Favor blend of MOS
guidance (upper 50s northwest to low-middle 60s se). More extensive/longer
duration rain will result in cooler temperatures by 3-4 degrees.
Sunday night...model guidance has gained some consensus on a
slightly more pronounced perturbation and attendant surface wave to
affect our region Sunday night...especially after midnight. Main
lifting mechanism appears to be from a 140+ upper jet lifting northward
across the northern middle Atlantic...placing central NC in the
favorable right entrance region. This may lead to rainfall amounts
on the order of a half inch over the Piedmont. Plan to trend probability of precipitation
from chance in the evening to high likely late Sunday night into the
first part of Monday morning. Lows in the middle 40s northwest to lower 50s southeast.
Long term /Monday through Friday/...
as of 325 PM Friday...
Monday through Friday... perturbation aloft and associated surface
wave and front will cross central NC Monday morning. West-northwest flow
behind the exiting weather system Monday afternoon-night will lead
to drier and more stable conditions. Based on current model
timing...favor highest probability of precipitation Monday morning with decreasing probability of precipitation
Monday afternoon and less than 20 percent probability of precipitation by Monday night.
Temperatures Monday dependent upon eventual track of the surface low. Some
models favor a track closer to the northwest Piedmont...placing bulk of County Warning Area
in the warm sector. Other models cluster around a solution with the
surface wave crossing SC then up along the NC coast...placing central NC
in the cool sector. The later solution seems more likely considering
the potential for a shallow layer of cool air to advect into the
region behind the initial perturbation. Have trended maximum temperatures a
little cooler than MOS guidance but later forecasts may need to
Tuesday through Friday will feature mainly dry weather with a
cooling trend as broad upper level trough covers the U.S. The westerly
flow aloft and surface high pressure diving southward across the central
U.S. Normally signals dry and seasonably cool conditions for our
region. Maximum temperatures by the New Year will average in the 40s to near 50
with min temperatures in the 20s to around 30.
Aviation /00z Saturday through Tuesday/...
as of 620 PM Friday...
24-hour taf period: high confidence that VFR conditions will prevail
through the current taf period. Winds should be calm to light and
variable overnight as high pressure builds overhead. Skies will be
clear to partly cloudy overnight...with high clouds building in
Saturday morning...becoming scattered to broken. Winds will be
light and generally southwesterly on Saturday. -Kc
Looking ahead: expect an increasing potential for MVFR/IFR ceilings
and rain late Saturday night through Monday as a slow moving front
progresses across the region. Conditions are expected to improve by
late Monday night into Tuesday. -Krd