Quantitative Precipitation Forecast
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FXUS04 KWBC 120918

Quantitative Precipitation Forecast Discussion
NWS Weather Prediction Center College Park MD
517 AM EDT Mon Mar 12 2018

Final Day 1, Day 2 and Day 3 QPF Discussion
Valid Mar 12/1200 UTC thru Mar 15/1200 UTC
Reference AWIPS Graphics under...Precip Accum - 24hr

Day 1...

...Mid Atlantic into the Northeast...

A strong compact shortwave is centered over TN early this morning.
This wave will continue moving southeast through the day...with an
area of rain/snow associated with it. During the morning hours the
developing coastal low will become the dominant low...intensifying
as it moves northeastward off the Mid Atlantic coast. However
still expect a well defined area of lift to accompany the mid
level wave as it moves into the Carolinas...with a QPF maximum to
the north northeast of this feature. For this aspect of the system
(QPF across the Mid Atlantic) favored a blend of the latest
GFS/UKMET/HREF/NBM which seemed to offer a good median solution.
This shrinks/decreases the area of QPF over portions of VA and
northern NC...which should see the combination of downslope flow
east of the Appalachians and the mid level wave tracking further
southeast...both help cut down on amounts. QPF totals over central
and eastern NC and SC remain pretty similar to our previous
forecast...just a bit more added detail.

Some model differences were noted with the strength and track of
the developing surface low...which should really begin to
intensify tonight as multiple mid level waves interact with it.
Overall thought the 12z ECMWF was too weak and thus too suppressed
with QPF through the day 1 period ending at 12z Tuesday. Meanwhile
thought the 0z NAM was likely too strong and thus too far
northwest and heavy with its QPF. For now preferred to stay in the
middle of this model spread. A blend of the 0z GFS/UKMET/HREF mean
and the 03z NBM seemed to represent a good middle ground solution.
This blend also ends up pretty close to what the older 0z ECMWF
from last night was depicting. This blend brings the 0.5" line up
to around Boston...with 0.75"-1" over portions of far southeastern
MA on to Cape Cod. Additional QPF will occur after 12z...thus see
the day 2/3 portion of the discussion or our heavy snow discussion
for more details regarding the remainder of the storm.


Precipitation will move into northern CA late tonight as a cold
front moves ashore. Given the well defined nature of the front and
good mid/upper level forcing forecast...would expect to see a
broad area of steady precipitation move ashore. Rainfall rates in
the 0.3"-0.4" range in an hour appear likely across the favored
terrain of northern CA into the Sierras. Model agreement was
generally good...allowing for a multi model blend for QPF. This
keeps good continuity from our previous forecast...just bumps
amounts up a tad over the Sierras.

...South Florida...

Scattered convection will remain possible over south FL into mid
afternoon ahead of the southward dropping cold front. A decent
amount of model spread was noted...but at this point not expecting
anything too organized too occur. Thus the WPC QPF ends up pretty
close to the 0z HREF mean...with a broad area of 0.25"-0.5"
amounts. Given the anomalous PWATs in place...will probably see
some locally heavier totals...but at this point anticipate those
will be isolated in nature.

Days 2/3...

...The West...

A deep long wave trough axis extending south from AK with
vorticity maxima centered off northern CA comes ashore Tuesday
pushes inland through the midweek, with a closed embedded upper
low approaching WA Wednesday night. Multiple shortwaves wrap
around this low, digging the trough axis to southern CA and
providing forcing to the plume of 0.75 inch PW moisture that
streams inland ahead of the axis by Tuesday morning. Heavy precip
will be centered on the CA Cascades and Sierra Nevada Tuesday into
Wednesday with moderate intensity precip spreading south along the
central and southern CA coast east Tuesday, and reaching the
Rockies late Wednesday. This is a multi-faceted complex of
activity moving in with understandable differences in guidance.
WPF QPF was based on a blend of the 00Z GFS/ECMWF with a touch of
00Z UKMET and consideration to the previous forecast.

The heaviest precip could exceed four inches across both days is
expected at higher elevations of CA which will be in the form of
snow in the crashing height falls. See QPFHSD for further snow


Better agreement is seen among the 00Z global guidance suite
compared to recent days for the phased low already developed off
the southern New England coast Tuesday morning. The 00Z ECMWF
returned farther east and strengthened which is in line with the
preferred guidance consensus. WPC QPF is based on the 00Z
ECMWF/GFS with a bit of the 00Z UKMET and consideration of the
previous forecast.

As the surface low nears offshore New England, there is a clear
model signal for a deep layered frontogenesis arcing from
south-southwest to north-northeast over southeastern New
England/MA through the day Tuesday. Strong banding with potential
convective elements and ocean enhancement appear likely. A
strengthening deformation axis should impact eastern and northern
New England while light to moderate snow falls farther west into
the interior Northeast. Orographic influences are also expected as
the cyclonic flow passes offshore.

The larger scale upper low will persist over the eastern Great
Lakes Tuesday before ejecting east and arcing north across New
England Wednesday night. This low will ensure plenty of cold air
to support lake effect/enhanced snow showers downwind of the
eastern Great Lakes with a focus southeast of Lakes Erie and
Ontario and wrap around moisture into northwestern New England.
Further info on snowfall can be found in the QPFHSD discussion.

...Big Bend of Texas...

Anticyclonic flow around a surface high building into Louisiana
Wednesday allows one inch PW (2 to 3 standard deviations above
normal) to push up mainly the Mexican side of the Rio Grande. The
combination of upslope flow and peak diurnal heating allows around
a quarter inch areal QPF Wednesday afternoon which is favored more
in the 00Z GFS than the 00Z ECMWF. Neither model features
instability...so there is no risk for flash flooding here.


Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml


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