Quantitative Precipitation Forecast Discussion
NWS Weather Prediction Center College Park MD
153 PM EDT Thu Mar 15 2018


Prelim Day 1 QPF Discussion
Valid Mar 16/0000 UTC thru Mar 17/0000 UTC
Reference AWIPS Graphics under...Precip Accum - 24hr


Day 1

...Western U.S...

An upper low digging southward along the northwest U.S. coastline
will eject a couple shortwave into the West through the Day 1
period. Low-level onshore flow of 20 kt will provide the moisture
and some (relatively modest) orographic lift. Some enhancement of
precipitation rates is likely, however, due to fairly steep height
falls  advancing into CA during the period, and cold temperatures
aloft even resulting in some areas of modest convective
instability. The best combination of these three factors appears
to be across northern/central CA, and this is where the heaviest
precip is forecast on Day 1. Liquid equivalent amounts exceeding a
half inch are expected over the northern CA coastal ranges, while
the heaviest amounts (1-2 inch liquid) are expected across the
northern/central Sierras. Much of this precip will fall as snow.
Please refer to products issued by the Winter Weather Desk for
further details on the winter weather threat. Farther inland,
relatively lighter amounts are expected given relatively poor
moisture flux ahead of this system, but some convective
enhancement appears possible across the Great Basin as well. Areal
averaged QPF amounts of a quarter to perhaps locally reaching a
half inch are expected. WPC QPF was based on a multi-model blend
including the WRF-ARW/NMM, ECMWF/GFS, and in-house bias corrected
ensemble guidance.


...Central Rockies/Central Plains...

Mid/upper-level shortwave crossing the Rockies this evening is
expected to eject into the central High Plains tonight. Models are
in good agreement that this feature will quickly intensify as the
vortex stretches, with a surface low expected to rapidly spin up
near the CO/KS border late this evening into the overnight hours.
The upper wave will initially produce generally light precip
across portions of the central Rockies tonight, before tapering
off by Fri morning as the best energy shifts into the plains. As
the upper wave amplifies and the developing surface low quickly
deepens, a dynamically forced band of precipitation is expected to
develop on the north side of the system, from eastern WY east
across the plains. The warm conveyor belt should begin to quickly
ramp up tonight as strengthening low-level flow develops across
the southern/central plains ahead of the developing system. A
50-60 kt low-level jet shown by the GFS by 06-12Z Fri would result
in a fairly impressive area of moisture flux convergence in a
west-east oriented band centered roughly along the SD/NE border,
with even some modest elevated instability making it into the
region. The Day 1 QPF shows a max across this area, with the
heaviest QPF amounts exceeding 1.25" possible. Another max is
expected across southeastern WY where low-level upslope
enhancement will likely occur. Snow and freezing rain are expected
across portions of this area. Please refer to products issued by
the Winter Weather Desk for further details on the winter weather
threat. WPC QPF was based on a multi-model blend including the
WRF-ARW/NMM, ECMWF/GFS, and in-house bias corrected ensemble
guidance.


...Gulf Coast to lower/mid MS Valley...

Farther southeast, increasing moisture and instability will
support areas of convection focused along the cold front from the
mid-Mississippi valley to the Gulf Coast. Pwat values in the
1-1.25 inch range along with expected decent forward movement
(owing to 40-50 kt mid-level flow) should keep overall amounts in
check. The heaviest amounts are expected across portions of
central/southern MS, where areal averaged amounts exceeding a half
inch are expected, with localized amounts potentially nearing 1
inch in the heaviest areas. WPC QPF was based on a multi-model
blend including the WRF-ARW/NMM, ECMWF/GFS, and in-house bias
corrected ensemble guidance.


...Northeast...

Northwesterly flow will favor certain terrain and lake-enhanced
areas of New York and New England for additional 0.10 inch or
greater liquid equivalent tonight into Friday morning. Rising
upper heights and decreasing moisture after 12Z Fri should begin
to suppress shower activity.

Ryan