Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Raleigh NC
1026 am EDT Tuesday Jun 2 2015
Synopsis...a middle level low pressure trough and surface cold front
over the Tennessee Valley will move slowly eastward then stall over
the Carolinas through the work week... bringing unsettled weather.
Near term /through tonight/...
as of 1015 am Tuesday...
Upper level low pressure over the Tennessee Valley and surface high
pressure in the western Atlantic have combined to bring wet
southerly flow out of the eastern Gulf of Mexico and up into the
area ahead of a near stationary cold front in the western part of
the state. Different from prior days...many indications pointing to
the eastern half of the County Warning Area being more favorable for convection this
afternoon/evening as opposed to the northwest Piedmont which has seen the
majority of the action over the past several days. A couple reasons
for this. First...Water vapor and upper air analysis at multiple
levels point to drier air advecting into the southwest Piedmont with
the exception of 700 mb which still shows dewpoint depressions less
than five. Above and below this level...conditions are much drier as
opposed to saturated to the surface which we have seen in recent
days. Second...pre existing showers this morning have helped to keep
temperatures down which will limit our instability over the western
half of the County Warning Area this afternoon. Finally...the southern half of the
cold front has moved a little more eastward...and thus will push the
best moisture from the south...further east. Differential heating
boundary very clear in the observations just east of the Triangle
with temperatures in the upper 60s to lower 70s to the west and upper 70s
to near 80 degrees in the east. This will become the focus for
convective initiation later in the day and therefore believe the
best chance for significant coverage of convection to be along the I-
95 corridor. Storm Prediction Center has the area in a marginal risk of severe weather with
wet downbursts being the most likely threat. Rap forecast really
agreeing with this scenario. Do not think we will see widespread
severe storms but shear is better than previous days so certainly
think it is in the realm of possibility. Highs in the upper 70s west
to middle 80s east.
Tonight expect showers and storms to linger into the early morning
hours in the east with not much west of Highway 1. Lows in the low
60s west to upper 60s east. Expect good chances for fog/low stratus
across the northern half of the County Warning Area.
Short term /Wednesday and Wednesday night/...
as of 315 am Tuesday...
Atmosphere changes little Wednesday/Wednesday night due to the
nearly stationary upper low to our west and the ridge to our east.
Lobes of vorticity rotating around the low will cause periods of
showers and storms with coverage enhanced by afternoon heating.
Similar to today...rainfall rates of 1-2 inches per hour may lead to
nuisance flooding of streets/low lying areas. Since flooding is
expected to be minor and limited in scope...not anticipating the
need for a Flood Watch at this time. Near surface northeasterly flow behind the
slow moving surface cold front that should in vicinity of Highway 64
will advect slightly cooler air into the northern counties. This low
level cold air advection along with periods of showers and extensive cloudiness
will result in high temperatures in the 70s over most of the region with
near 80/lower 80s confined to the far south-se.
The threat for showers and storms will continue into Wednesday night
though coverage should begin to wane after 03z-05z as atmosphere
undergoes usual nocturnal stabilization. Min temperatures lower 60s north
to middle 60s south.
Long term /Thursday through Monday/...
as of 250 am Tuesday...
Thursday through Friday night: surface high pressure is expected to
extend southward and into portions of central NC still on Thursday.
However... the center of the high is expected to be moving off the
northeast U.S. Coast on Thursday. This should yield a moderating
airmass some. However... with plentiful moisture and a lingering
northerly flow over most of the area expect we will once again see
below normal temperatures across central NC... with highs ranging from the
lower to middle 70s northwest to the lower 80s southeast. The middle level low on
Thursday will slowly wobble eastward from the NC mountains and
eventually make it to the coastal plain on Friday. This should
continue to give US a good chance of showers and storms on Thursday
into Thursday evening (scattered to numerous)... with the better
chances on Friday across the eastern half of the forecast area as
the middle level low moves across the County Warning Area. However... overall deep
shear will remain weak... with surface instability weak to modest at
best. Thus... any severe threat should remain isolated during this
time... with localized flash flooding (especially in urban areas and
locations that happen to receive several rounds of heavy precip)
being the main hazardous weather threat during this time frame.
Temperatures should rebound a bit on Friday with a continued moderating
airmass... with highs generally in the lower 80s. Lows Thursday and
Friday night generally in the middle 60s.
Saturday through monday: unsettled forecast continues into next
weekend. With the passage of a northern stream disturbance to the
north of the area on Friday night into Saturday... another trailing
or backdoor cold front may again sink southward into central NC.
This combined with a continued weak middle level trough remaining
across the region and possibly additional short wave energy dropping
southeastward and into the back side of the weak middle level trough
over the area (as shown on the latest ecwmf... which would enhance
coverage of showers and storms even more) should result in at least
chances for afternoon and evening showers and storms. Still a little
to early to tell how far south this possible front will make it.
Thus... will continue to stick close to wpc for temperatures...with high
temperatures near to slightly below normal for early June and lows near
Yet another disturbance is expected to track from the central and
northern plains on Saturday into the Great Lakes region by late
Sunday into Monday morning. This will eventually send another front
towards central NC late Monday from the west. Thus... will continue
to keep chance probability of precipitation in for day 7 in the forecast with near normal
Aviation /12z Tuesday through Saturday/...
as of 735 am Tuesday...
A very moist air mass over the Carolinas coupled with the approach
of a low pressure system and its attendant surface cold front will
result in adverse aviation conditions across much of central NC
through Wednesday night.
This morning...showers will become a little more numerous over the
Piedmont and possibly over the sandhills as a perturbation aloft
lifts northward across the region. These showers and isolated storms will
cause periods of MVFR ceilings/visibility. Heating of the moist and
slightly unstable atmosphere this afternoon will cause numerous
showers and scattered thunderstorms to develop with the greatest
concentration expected over the eastern half of central NC. This
batch of convection may impact krdu...kfay and krwi.
The areas of convection will be slow to diminish tonight as a surface
cold front drifts southward into our area. This feature will likely
aid to maintain a few showers and storms well into the night.
The unsettled weather pattern will continue Wednesday into Thursday
as the atmosphere will maintain its moist and slightly unstable
character. This will cause periods of MVFR/IFR conditions due to
ceilings and convection.
Conditions will begin to slowly improve Friday and Saturday as the
atmosphere becomes slightly drier. This will lead to fewer showers
and storms during the heat of the afternoon-early evening.
However...there is still a good probability of early morning
MVFR/IFR ceilings and visibility both mornings.