997
fous11 kwbc 121016
qpfhsd
probabilistic heavy snow and icing discussion
NWS Hydrometeorological Prediction Center Camp Springs MD
515 am EST sun Feb 12 2012
valid 12z sun Feb 12 2012 - 12z Wed Feb 15 2012
Day 1...
...Central-southern Great Basin/rockies and adjacent plains...
A mid-upper trough drifting east from the Great Basin will make
its way east across the central/southern rockies by the end of the
period. Strongest h25 winds should remain on the eastern side of
the wave...with speeds increasing to around 130 kts once the jet
makes its way onto the plains. Until the system reaches the
central/southern rockies Front Range...pcpn should be driven by
difluent flow aloft combined with enhancement from orographic lift
in the low levels. Then there should be a mixed bag of pcpn over
the adjacent High Plains given a fair amount of lift once the mid
level flow rides up and over an Arctic air mass east of the mtns.
Gave more weight to the GFS/ECMWF. Thus we ended up with highest
probabilities from the Wasatch to the San Juans with the main
system. We also highlighted a risk of 4 inches of snow mainly in
the TX Panhandle to SW OK should enough moisture arrive in time.
Some risk of icing was also indicated out here due to the
potential for some rain to fall while the Arctic air mass is still
in place at the sfc.
Day 2...
Central to south Central Plains and MS valley...
As the mid/upper level system emerges over the plains...a center
of Arctic high pressure should finally be easing eastward into the
OH/TN valley. Increasing southerly flow at the sfc ahead of the
wave should result in pcpn becoming more widespread over the MS
valley by 13/18z and continuing through the remainder of day 2.
Qpf amounts are generally expected to be light...with
corresponding snowfall amounts generally below 4 inches. However
we opted to insert a low end slight risk /roughly a 1 in 10
chance/ of 4 inches since snow to water ratios should be fairly
high and it would not necessarily take a large change to produce
some amounts at or marginally above 4 inches.Of bigger concern is
potential for icing mainly in AR to extreme southern MO...and
extending west and south once warm advection kicks aloft. Since
precipitation types are tricky...opted to remain in the slight
category. The continuing warm advection should result in snow
changing to a mixture of sleet/freezing rain and then eventually
rain.
...Pacific northwest/CA...
The models remain in agreement on the development of a 700-300 mb
wave in the northeast Pacific and for this wave to move southeast
across the coast of or/CA and inland. As the wave
approaches...warm and moisture advection develop. Difluent flow
aloft in advance of the approaching upper trough favors periods of
upper divergence to support ascent across the or-CA
Cascades/Siskiyous/shastas/northern and central Sierra. Steepening
lapse rates in the Post frontal air mass support snow showers
continuing in the or Cascades. Thus we continued the idea of a
high risk of 4 inches in parts of the sierras eastward into NV
where falling heights lead to dropping snow elevations...and good
multi overlap of multi-model/09-15z sref mean qpf maxima.
Day 3...
...NV/northern AZ/UT/co...
The models show the next upper trof / closed low moving slowly
southeast from NV to UT/co. Upper divergence maxima in the left
exit region of the jet cross UT/co/nm should be coinciding with a
band of convergence ahead of and along the 700 mb front as it
progresses east across the region resulting in periods of snow.
Snowfall amounts are expected to be enhanced where pre-frontal
southwest flow turns upslope in terrain facing the southwest flow.
Consequently...higher amounts and probabilities are forecast in
parts of the UT Wasatch in the San Juans...wit a secondary maxima
developing in the AZ Mogollon Rim. Gave more weight to the UKMET
in this area.
The probability of significant icing is less than 10 percent.
Bann
$$
