Scientific Forecaster Discussion
Area forecast discussion
National Weather Service State College PA
1141 am EST Thursday Jan 29 2015
An Alberta clipper will cross the area this afternoon and
tonight. High pressure will return for late Friday into Saturday.
A new storm may bring a round of wintry weather for later Sunday
into Monday...before another surge of cold air works into the area
early next week.
Near term /until 6 PM this evening/...
Clouds continue to thicken over the region as of late
morning...with the first precipitation reaching the ground over
far western PA in the form of some sleet or freezing rain.
The main area of precipitation is shown by the hrrr to be entering my
western zones by 18z/1pm. Still some concern for a brief period of
light freezing rain or sleet at the onset...even outside the
current advisory areas. Snowfall totals in our gridded forecasts
look reasonable and we are not expecting more than a trace of
ice...but if it should coincide with the evening commute untreated
roads could pose travel problems. At this stage we plan to handle
this expected brief period of ice with special weather
statements and nowcasts...and will monitor for the possible need
for a headline product.
A strong shot of warm air advection aloft and moderately strong upper level
divergence associated with the right entrance region of an upper
level jet maximum across southern Ontario will lead to periods of light to
briefly MDT snow focused across the northwest half of the state this
afternoon into tonight.
Guidance is consistent in showing warm air surging in aloft
today...then being pinched off quickly as precipitation develops and the
column evaporatively cools. However...the rap/hrrr hourly data
show the warm air actually pushing back north between 21-00z over
the southwestern zones...which if it coincides with ongoing precipitation will
complicate the precipitation type scenario over The Laurels and some
Maryland border counties. The potential for the wintry mix is the
low probability outcome at this Point.
Rest from earlier...
Model consensus remains unchanged to a large
degree with respect to the quantitative precipitation forecast from the clipper system moving by
just to our north across lakes Erie and Ontario late today and
tonight. A general 2-4 inch snowfall is likely over the
alleghenies of western Penn where a Winter Weather Advisory is in
effect from late this morning through 12z Friday. The Central
Ridge and valley of the state will 1-2 inches by midnight
tonight...while counties throughout the lower susq valley receive
generally less than 0ne inch. A bit of sleet and even patchy ice
from light freezing rain is possible across southern Penn late
today and this evening as the rather shallow wedge of warmer air
aloft drifts east.
Highs this afternoon will reach the upper 20s to around freezing
in the increasing/brisk SW flow ahead of the clipper/Arctic front.
Short term /6 PM this evening through 6 am Friday/...
the brunt of the current storm system will occur late this
afternoon and early tonight...with some brief periods of MDT snow
likely as one or more north/S bands of enhanced 850-700 mb uvvel
crosses the region. 03z sref indicates that an axis of anomalous
/2-3 sigma/ 850mb moisture flux and similarly anomalous /50 knots/
southwesterly low level jet will be gradually weakening as it crosses Penn between
21z and 06z Friday.
Surface cold front extending south from the low...pushes across our
western zones late this evening...then clears the susq valley
between 07-09z Friday. Snow will taper to snow showers by late
tonight...mainly in the isolated form across the central and
Low temperatures tonight will range from the middle to upper teens across the
western mountains...to the middle and upper 20s across the susq valley. A
slightly earlier timing of the cfropa will knock these values down
by a few degree f.
Long term /Friday through Wednesday/...
periods of generally lighter snow will linger into early
Friday...as the main cold front drops southeast.
A tight low level pressure gradient in the wake of the cold front will
bring gusty northwest winds and slowly falling temperatures across the
region during the daylight hours Friday. Wind gusts will be in
the middle to upper 20 knots range for most areas creating a wind chill
in the single digits to low teens in the susq valley....and 5
below to 15 below across the northern and western mountains
Friday night will be bitterly cold...as skies clear and winds die
down. Low temperatures will vary from the single digits below zero across
the north...to single digits above zero in the south.
A bright day on Sat...but not real warm.
Still have chance probability of precipitation on Sunday. A lot of spread in the model output
the last few days. 12z models backing off some now. Pattern
supports low pressure systems that are fast moving and weak. Would
still expect some light snow into Monday morning.
A cold night Monday night...as the next high builds into
Some hints at another fast moving low for middle week...for now
went with chance probability of precipitation.
Minor changes to the package made.
Aviation /15z Thursday through Monday/...
VFR will continue into early afternoon before rapidly
deteriorating to MVFR/IFR as snow overspreads the flying area
from west to east.
Snow will begin around or shortly after noon in Bradford. Expect
IFR conditions to develop rapidly with the snow...spreading east
Reduced conditions should improve overnight with the passage of
the cold front...around 06z Friday over the west. More like 09-12z
over the southeastern areas. The conditions at bfd and jst will likely
remain MVFR/IFR into Friday.
Sun...restrictions developing with snow overspreading from the
Monday...reduced conds early in snow. Improving from west to east
Winter Weather Advisory until 7 am EST Friday for paz004>006-
near term...la corte/Lambert