Quantitative Precipitation Forecast
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FXUS04 KWBC 161823

Quantitative Precipitation Forecast Discussion
NWS Weather Prediction Center College Park MD
223 PM EDT Wed May 16 2018

Prelim Day 1 QPF Discussion
Valid May 17/0000 UTC thru May 18/0000 UTC
Reference AWIPS Graphics under...Precip Accum - 24hr

Day 1...

---Eastern U.S.----

There is not expected to be any significant changes to the very
wet pattern affecting large portions of the eastern U.S.  A very
large area of much above average pw values---1.5 to 2.5+ standard
deviations above the mean will persist along and to the east of
the elongated mean trof stretching from the OH Valley---into the
TN Valley and central to eastern Gulf Coast region.  Widespread
showery conditions will persist from Florida--northward through
the Southeast---Southern to Central Appalachians---Mid-Atlantic
into southern NY state and southern New England.  The href mean
and in house hi-res mean qpf was used primarily for qpf details
day 1 to mitigate the typical run to run and model to model qpf
detail differences.  The combination of the much above average pw
values and broadly difluent upper flow to the east of this mean
trof will support widespread moderate to heavy totals across these
areas.  Focus areas for the heaviest precip likely through the
Southern to Central Appalachians from northeast GA---the Upstate
of SC---western NC---western VA into eastern WV where uvvs will be
enhanced by topography.  The persistent west to east oriented
stationary front between the OH and TN valley---into the
Mid-Atlantic will also help focus a potential heavy precip axis
through the Mid-Atlantic into far southern NY state and coastal
southern New England.  Excessive rainfall wise---a broad marginal
risk area is maintained across these regions---with slight risk
areas along and north of the stationary front from the Central
Appalachians into the Mid-Atlantic and over the Southern
Appalachians in the vicinity of the above mentioned precipitation

...Lower MS Valley into the Central Gulf coastal region...

Height fall expected to drop southeastward on the back side of the
mean trof position stretching from the OH-TN Valleys into the
Central Gulf coastal region.  PW values are not as anomalous in
this area as points to the east of the above mentioned mean trof
position---generally around 1 standard deviation above the mean.
Convection enhancing over the Southern Plains this afternoon ahead
of these height falls expected to weaken by the beginning of the
upcoming day 1 period as they push toward the Lower MS Valley.
This activity will likely re-ignite later day 1--especially
Thursday afternoon over the Lower MS Valley-Central Gulf coast
region.  There is a lot of model qpf spread here---leading to low
confidence.  Moderate areal average amounts depicted with the
likelihood of locally heavier totals where convection does become
most organized.

...Northern California...Great Basin...

Models are similar day 1 with the eastward push of the north
central California closed low moving into the Great Basin.
Showers will remain active to the east and northeast of this
closed low from northern California---eastern Oregon---northwest
NV.  This will support moderate to isolated heavy precip totals
across these areas where pw values are expected to remain much
above average---2 to 2.5 standard deviations above the mean.  No
changes planned to the marginal risk area depicted across these
areas to the north and northeast of this closed low.

...Northern Rockies...

A farther northeast closed low will be moving slowly northward
from the Northern Rockies into the northern High Plains this
period.  Shower activity likely to continue to the north and
northwest of this system---enhanced by developing north
northeasterly upslope flow into the northern Rockies.  Moderate to
heavy totals likely from northern Idaho into northwest MT.

...Northern Plains into the Upper MS Valley...

Shortwave energy pushing to the northeast of the Northern Rockies
closed low will enhance uvvs along and north of the west to east
oriented front forecast to lie across the Northern Plains into the
Upper MS Valley.  The best chance for shower development along and
north of this boundary will be Thursday afternoon ---with this
activity likely to enhance further after the day 1 time period
into the beginning of the day 2 time period.  For the day 1
period---the timing of the convective development will likely keep
the areal extent of potentially heavy rain fairly small---with an
expanding area likely day 2 heading into convective max time early
Friday morning.



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