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FOUS30 KWBC 170803
QPFERD
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
403 AM EDT Wed Jul 17 2019

Day 1
Valid 12Z Wed Jul 17 2019 - 12Z Thu Jul 18 2019

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM THE SOUTHERN
GULF COAST TO THE GREAT LAKES REGION AS WELL AS A PART OF THE
DAKOTAS INTO MONTANA...

...Arkansas into northern Mississippi and western Tennessee...
Convective bands were continuing to spread eastward as of Tuesday
evening from Mississippi into Alabama while rain over the western
Tennessee Valley made its way into the southern Great Lakes region
during the day.  Most hi-res models indicate the redevelopment of
convection very late in the period (03-06Z or later) after a lull
in activity. As we can't rule out some instability recovery in an
intervening period of several hours, and the air mass will remain
extremely moist with PWs in excess of 2 inches, this still seems
to be a plausible scenario and no significant changes were made to
the Slight Risk area previously issued. Given persistent W-SW
low-level flow and an upstream position of a pool of stronger
instability, backbuilding and training would certainly be possible
with any convective development. .

...Mid Mississippi and Ohio Valleys into New York, Pennsylvania
and the Mid Atlantic...
A Slight Risk was maintained in association with Post-Tropical
Cyclone Barry's moisture plume from the Mid Mississippi Valley
into the eastern Great Lakes. No changes were made to the Marginal
Risk which was expanded earlier in the day across parts of central
Pennsylvania, Maryland, eastern West Virginia and eastern
Virginia. Although hi-res models still indicate relatively
progressive bands of convection this afternoon in the Mid
Atlantic, the 12Z HREF has some low probabilities of 2 inch per
hour rain rates. This would likely be sufficient to cause some
sporadic flash flooding, even if convection is moving along at a
quicker pace.

...Central and Northern Plains...
A Slight Risk was maintained and even expanded across portions of
the Northern High Plains from SE MT into parts of North Dakota and
South Dakota.  12Z runs of the ARW and NMMB both show a decent
swath of 1 to 2 inch amounts moving out of Montana and spreading
across South Dakota overnight.  This is in a region where the
global models forecast enhanced low level convergence, instability
and frontogenesis.  Given that Precipitable Water values were
around 1.25 inches in the western part of the state...which was
over 1.5 standardized anomalies greater than climatology and over
the 75th percentile...swaths of heavy rainfall appear likely.
Antecedent conditions will make the region more vulnerable to
flash flooding as 30-day rainfall is over three times normal
values and flash flood guidance is substantially reduced -- in
some areas as low as 0.8 to 1.0 inches in one hour.

Bann


Day 2
Valid 12Z Thu Jul 18 2019 - 12Z Fri Jul 19 2019

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS THE
NORTHERN GREAT LAKES REGION...

...Northern Wisconsin/Northern Michigan...
At the start of the forecast period, amplifying shortwave at the
nose of unseasonably strong 250mb jet that is flat and stretches
well back into the Northeast Pacific will be lifting out of the
area into Ontario, with a strong surface low pressing the
attendant frontal zone across N MN/Lake Superior.  However, given
the orientation of the deeper flow out of the southwest veering
slight west, the frontal zone flattens out across the UP/N WI into
south central MN.  The 850mb LLJ will be strong across the region
at over 40kts and with solid moisture convergence at the
inflection where the front flattens will support total PWats AOA
2".   The LLJ will continue to be strong throughout day with
slight backing supporting moisture flux anomalies over 4 Std.Dev
from normal.

However, there will be a ongoing convective complex across the
Upper Mississippi River Valley and there is growing uncertainty in
its evolution on D1.  Hi-Res CAMs suggest with high instability
and strong LLJ, propagation vectors will have a stronger southerly
component potentially across NW IA/N IL into NW IND.  Global
guidance is quicker to kill off the MCS and favor stronger
ascent/forcing near the front and therefore supports stronger
convection across N WI into the UP and Northern Portions of the LP
of MI.  The former would disrupt warm advection of necessary
moisture/instability across Lake Michigan for the build up for a
complex Thursday evening into Friday morning.

Given the uncertainty in the placement, but the very strong
synoptic environment for heavy rainfall clearly capable of
producing heavy rainfall rates and short-term totals capable of
scattered flash flooding conditions warranting a Slight Risk.  At
this time, given the stronger larger scale signals will favor the
global guidance slightly in the placement of the Slight Risk,
however, will broaden the Marginal Risk of excessive Rainfall
further south and west to account for the backbuilding system.


...New Jersey/NYC...
By the start of the forecast period, the deep moisture conveyor
belt associated with Post-Tropical Cyclone Barry has pressed
eastward offshore into the Northwest Atlantic.  However, its
remaining sheared shortwave that supported a new surface
circulation out of the Mid-Atlantic lee surface trof on Wednesday
(not PTC Barry) will be in the vicinity of the Mid-Delaware River
Valley, maintaining some near surface moisture.  This will result
in a tightening deep moisture gradient with 1.75-2" remaining
through the total depth by midday.  Given a slight trend for
eastward press of the moisture, remaining high clouds are likely
to eastward as well allowing for increased insolation and
instability to build by early afternoon across New Jersey.  Hi-Res
CAMs and even global models suggest SBCAPEs to exceed 1000 J/kg
perhaps reaching as high as 2000 J/kg.  As such there is a solid
clustering of Hi-Res CAMs suggesting 1-3" totals are possible with
slower moving/merging convective cells in the afternoon.  As such
have maintained a Marginal Risk of Excessive Rainfall for NJ and
the low FFG areas near the NYC metro, but have pulled the Risk
area out of southern New England where lingering
clouds/precipitation is more likely to limit sufficient
instability to build back (additionally higher FFG values).

Gallina


Day 3
Valid 12Z Thu Jul 18 2019 - 12Z Fri Jul 19 2019

...The Probability of rainfall exceeding flash flood guidance is
less than 5 percent...

...Upper Mississippi Valley into Northern Great Lakes...
Exceedance of flash flooding is highly contingent on the placement
and evolution of multiple convective complexes expected on day 1
and day 2 lowering Flash Flood Guidance values across of
convection.

A frontal zone will remain stationary through day 2 into day 3,
pooling moisture and therefore instability throughout these few
days.  By day 3, moisture flux convergence will reach a general
lull given the LLJ will start to orient more parallel to the
boundary.  While there will be sufficient moisture (up to 2" total
PWats across S MN into N WI), the unseasonably strong polar jet
will be fairly flat oriented across the region, reducing upper
level shortwave/PVA support. However, convection that does develop
will have the tendency to move east along the frontal zone
reducing flash flood guidance for any further upstream development
that would have the tendency to repeat/train.  Currently, the
global guidance suggests the best corridor would be from SE MN to
the northern LP of MI, but given higher probabilities of
convection further south and/or west on D1 and D2 it is hard to
support placing a precise Marginal Risk at this time, but will
likely change as hydrologic/FFG situation becomes better realized
in the coming day or so.

Gallina




Day 1 threat area: https://www.wpc.ncep.noaa.gov/qpf/94epoints.t...
Day 2 threat area: https://www.wpc.ncep.noaa.gov/qpf/98epoints.t...
Day 3 threat area: https://www.wpc.ncep.noaa.gov/qpf/99epoints.t...


$$




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