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FOUS30 KWBC 241950
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
349 PM EDT Wed Jul 24 2019

Day 1
Valid 1947Z Wed Jul 24 2019 - 12Z Thu Jul 25 2019

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE SOUTHWEST...

...Southwest into the Southern Great Basin...
Expanded the Slight Risk northward and westwardinto parts of
southeast California as well as expanding the coverage into parts
of Nevada, northern Arizona and far southwest Utah for the
remainder of the afternoon and evening.  Convection was already
beginning to form by early afternoon over portins of California in
a region that was immediately downstream of a well defined
vorticity maximun located in the southwest corner of Arizona.  A
separate vorticity max in northern Arizona was visible in
satellite imagery.  These two features should help trigger and
focus convection as they move into a moist atmosphere that should
becoming increasingly unstable during the afternoon.  The 18Z
soundinig from 1Y7 already had a nearly saturated layer from
roughly 600 mb and upward and a Precipitable Water value of 1.9
inches. The area sounding from this morning and the 18Z sounding
from 1Y7 were also characterized by light flow, suggesting slow
cell movement. Thus...felt an expansion northward and westward was
in order.  Did not expand the Slight Risk too far into Arizona
given the subsidence behind the southwest Arizona vort max and
attendant trough extending towards the northeast.  The HREF
probabilities for 1 and 2"+ amounts remain focused along the
Peninsular Range of Southern California...western Mogollon Rim of
AZ...the southern Sierra...through the Wasatch and the Southern
Rockies of north central NM into southwest CO.  No fundamental
changes were made to the broad Marginal risk area which
encompasses convection tied closer to the terrain and any away
from it. -Bann

...Northern Florida...
The surface cold front presently 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 precipitable water values (PWs) of 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 effective bulk
shear starting out at 25 knots in this area, which could organize
convection, and ML CAPE of 1000-1500 J/kg, which would rise
towards 2500 J/kg this afternoon.  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.  Activity currently in the
northern peninsula should sink south and eastward, forward
propagating into an ML CAPE of 2000-3000 J/kg.  A marginal risk
was adjusted slightly southward to account for instability and
radar trends.

...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.

Roth/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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