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FOUS30 KWBC 240811
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
411 AM EDT Wed Jul 24 2019

Day 1
Valid 12Z Wed Jul 24 2019 - 12Z Thu Jul 25 2019

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS NORTHERN
FLORIDA...THE SOUTHWEST AND PORTIONS OF THE NORTHERN PLAINS...

...Northern Florida...
The surface cold front presently sinking through the Southeast is
expected to become stationary across North Florida on Wednesday.
Westerly low level flow along and to the south of this front off
the northeast Gulf of Mexico will keep pw values 2.00 to 2.25"+ in
the vicinity of the front over North Florida through this forecast
period.  This region will remain in the favorable right entrance
region of the upper jet along the east coast...with uvvs
accentuated in this high pw axis across North Florida.  There is
model consensus for the potential of heavy rains across North
Florida with training of cells in a west to east direction
possible.  Href neighborhood probabilities show 80-90%
probabilities of 2"+...50-80%+ probabilities of 3"+ and a few
areas of 20-40% probabilities of 5"+ amounts for the day 1 time
period.  A marginal risk was maintained from the previous issuance
with only a slight southward adjustment to match the model
consensus heavy precip axis.

...Southwest into the Southern Great Basin...
No significant changes expected to the position of the closed
upper high initially in the vicinity of the four corners.  PW
values 2 to 3 standard deviations above the mean will continue
around the peripheries of this upper high across the Southwest and
into the Southern Great Basin.  Expect another day of widely
scattered convection in this high pw axis.  Href probabilities for
1 and 2"+ amounts are focused along the Peninsular Range of
Southern California...Mogollon Rim of AZ...the southern
Sierra...through the Wasatch and the Southern Rockies of north
central NM into southwest CO.  However...convection removed from
the terrain is also possible...although confidence on any
convective placement is low.  A broad marginal risk was drawn to
encompass convection tied closer to the terrain and any away from
it.

...Far eastern Montana...western North Dakota and western South
Dakota...
Shortwave energy is expected to push eastward on the north side of
the Four Corners centered upper high...pushing out of the Northern
Rockies and into the Northern Plains.  This will support scattered
convection pushing east into the Northern High Plains late
Wednesday afternoon/evening.  Low confidence in qpf details..but
with increasing pw values...enhanced upper difluence and
strengthening boundary layer convergence ahead of these height
falls...isolated heavy precip totals are possible across this
region.

Oravec



Day 2
Valid 12Z Thu Jul 25 2019 - 12Z Fri Jul 26 2019

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF FLORIDA AND FAR SOUTHERN GEORGIA AS WELL AS THE FOUR CORNERS
REGION...

...Northern FL/Southeastern GA...
A stationary boundary remains draped across northern Florida which
will be the focus for continual shower/thunderstorm activity
through Thursday night. A tight gradient of 2+ inch precipitable
water will exist across this region in addition to marginal
instability. With the upper level jet streak just to the north,
the right entrance region will assist with large scale forcing
along with mid-level impulses moving atop the surface boundary.
Therefore, anticipate convection to move from the northeast Gulf
of Mexico across the northern FL peninsula through the period.
This will result in areal average precipitation values of .25-1+,
highest amounts across northwest FL.  Maintained the Marginal Risk
across this area given the expected precipitation activity from
Day 1 which may help soils become a bit more sensitive to
additional rainfall.  But then again, this is Florida where sandy
soils rebound quickly.  So do not expect anything more than very
isolated flash flood concerns, likely in more urbanized areas.

...Four Corners...
Mid/low-level monsoonal flow will persist across the Four Corners
region Thursday afternoon into the overnight as the mid-level
high/ridging provides moisture and impulses which when combined
with instability and upslope flow will promote convective
initiation along higher terrain. Continued light mean flow will
mean slow storm motions and the potential for the activity to
spread off the terrain into the plains. A trough crossing the
northern Rockies will promote additional lift, especially across
the higher terrain in CO/NM. Therefore, heaviest precipitation and
convective activity is expected across the Southern Rockies.
Moderate rainfall/rain rates are possible across the Black
Range/White Mountains. More modest precipitation coverage will
exist across the higher terrain in southern/central UT given the
flow regime. With more vulnerable soils and terrain across this
region, maintained the Marginal Risk.

Pagano/Campbell

Day 3
Valid 12Z Fri Jul 26 2019 - 12Z Sat Jul 27 2019

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS THE FOUR
CORNERS REGION AS WELL AS THE UPPER GREAT LAKES REGION...

...Four Corners Region...
Monsoonal moisture continues as the mid-level high/ridge axis
remains anchored across this region during the first half of the
forecast period.  The high starts to shift to the south and west
in time, with shortwave activity moving into the California coast
thus resulting in the Pacific moisture feed becoming shunted. The
moisture within the mid levels will be supplied by the
trough/front moving through the Central Plains. In addition,
mid-level impulses will shift farther south due to the retreating
high. Therefore, the best moisture/lift will be across southern CO
into NM. Precipitable water of 1.25 inches combined with pockets
of 2000+ J/kg will be available during the afternoon/evening
hours.  Thus, expect convection to initiate along the terrain and
move southwest with time. Given this, introduced a Marginal Risk
across this area.  Westerlies across UT and mid-level lift
combined with lingering moisture will be enough to promote some
convection across the terrain with another Marginal Risk area
noted.  Isolated flash flooding is expected with vulnerable soils
across this region.

...Upper Great Lakes...
Shortwave activity moving across Manitoba into Ontario will drape
a front across the Upper Midwest.  This mid-level trough will
become negatively tilted enhancing lift across this region.
Instability will surge with values over 2000 J/kg in some
locations.  This combined with precipitable water values of 1.5
inches aided by 30-40 knot low level flow should result in some
thunderstorm activity ahead of the front/shortwave.  Multiple
models illustrate this feature, though many have the strongest
forcing north of the International Border.  Given such uncertainty
and the potential for heavy rain makers, felt a Marginal Risk was
warranted.  If the highest QPF amounts shift north, then the
Marginal Risk will likely be removed.

...Gulf Coast...
A lot of uncertainty remains with respect to a potential tropical
wave of low pressure that could develop in the Gulf of Mexico
associated with a stationary boundary/lingering mid-level
vorticity. While there will likely be a non-tropical low pressure
that develops, there is uncertainty with respect to where, the
evolution, strength of the low and propagation.  With all these
elements of uncertainty, confidence is below normal across this
region with areal average precipitation mainly around a quarter to
near half an inch along the immediate Gulf coast, mostly from sea
breeze convection.  Therefore, did not introduce a Marginal Risk
at this time. We will continue to monitor.

Pagano


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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