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FOUS30 KWBC 262009
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
408 PM EDT Fri Jul 26 2019

Day 1
Valid 1720Z Fri Jul 26 2019 - 12Z Sat Jul 27 2019


...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL NEAR NEW MEXICO &
THE WASATCH MOUNTAINS OF UTAH...

...Central and Southern Rockies and Great Basin...
Afternoon convective development will occur over the CO/NM Rockies
following destabilization and upslope flow to
generate lift.  With monsoonal flow continuing, expect
precipitable water values to climb to 1.0 to 1.25 inches -- values
quite high for the region. This combined with moderate instability
will lead to locally heavy rain, perhaps as high as 2" an hour.
Areas with prior showers/storms have reduced 3-6 hr flash flood
guidance, between 1-1.5 inches on 3 hours, thus the need for a
slight risk area in the terrain of southeast CO across NM,
including the Sangre de Cristo and Manzano Mountains.

A shortwave trough moves east from California into Intermountain
West today.  As a result, the deep high centered over the four
corners region will shift south and west in time. This will send
further mid-level impulses around the ridge and help promote
widespread shower/thunderstorm development from over NV/UT due to
insolation within the disturbance's dry slot, currently shifting
northward across central NV and southern UT. Locally heavy rain is
possible where cell mergers occur, up to 2" an hour.  NAM forecast
soundings show low directional shear today, promoting localized
training.  An upgrade to a slight risk was coordinated this
morning with the SLC/Salt Lake City forecast office.


...Central Plains...
Morning thunderstorms across the central high Plains in the
vicinity of a wavy front have nearly dissipated.  Afternoon
convection is expected to form towards the western NE/SD border
which is expected to broaden around an after sunset and persist
until sunrise before fading thereafter.  Initially slow motions
could lead to locally heavy amounts before the activity attempts
to forward propagate, led by the activity towards the western
NE/SD border to the east-southeast near 30 knots.  Moisture,
instability, low-level inflow, and placement within a diffluent
1000-500 hPa thickness pattern suggest that a convective complex
is possible.  Used a blend of the guidance before making westward
adjustments to better match where the western fringe of the
low-level inflow intercepts the boundary.  Broadened the footprint
as such a complex would grow upscale.  Used the 12C isotherm at
700 hPa as a guide for its placement latitude-wise, which appeared
to work well with the morning activity.  The highest risk of heavy
rain-related issues lies in the northwest portion of the marginal
risk area, as the remainder mostly lies in the Sand hills of NE
where flash flood guidance is a bit higher.


...Upper Peninsula of MI...
A 700 mb shortwave rotating under the upper trough in southern
Canada induces a combination of well defined 700 mb convergence
and theta-e advection crossing from northeast MN through the UP of
MI this evening.  Convection generates in northeast MN during the
peak diurnal period and continues east southeast across the UP of
MI.  The high res WRF ARW/ARW2, NAM Conus Nest, and NSSL WRF all
forecast local amounts of 2-3" of rain, near flash flood guidance,
though only in a couple random locations.  Runoff problems could
stem from localized training in the area Fri night.  Broadened the
marginal area somewhat to account for the 12z mesoscale guidance
and newest version of WPC QPF.


...FL peninsula...
The 12z soundings indicated precipitable water values close to 2"
at Miami and Tampa/Ruskin, above average for July, with wet bulb
zero heights of 12,000-13,000 feet.  This degree of moisture
surrounds a weakening frontal boundary across the northern
peninsula.  Inflow at 850 hPa is fairly weak, on the order of 5-10
kts out of the southwest, which should allow for some sea breeze
penetration particularly closer to the ridge axis in southern FL.
However, a mid-level vorticity maximum is apparent in the model
mass fields and GOES-east imagery moving into northwest FL, which
per the SPC mesoanalyses is bringing effective bulk shear towards
25 kts.  Activity in northwest FL should move generally to the
east to east-southeast into the instability pool.  Any embedded
mesocyclones could yield higher hourly rain totals than would be
typical for forward propagating convection -- up to 2.5" an hour.
The outflow boundary from this convection looks like it will
augment rather than disrupt the daily sea breeze cycle due to its
timing.  The mesoscale guidance shows a signal for local 5"
amounts, but they tend to show a high bias in FL.  Nevertheless,
expect the higher rainfall totals today to lie in an arc from the
Everglades towards Daytona Beach given the position of the 850 hPa
ridge axis.  With greater than average moisture and some effective
bulk shear to work with which is not normally there in July,
thought it prudent to add a marginal risk of excessive rainfall to
the FL peninsula.

Roth/Petersen


Day 2
Valid 12Z Sat Jul 27 2019 - 12Z Sun Jul 28 2019

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE NORTHERN PLAINS...AS WELL AS PORTIONS OF COLORADO AND NEW
MEXICO...

...Northern Plains...
Moisture and instability increase ahead of a shortwave trough
crossing the northern Plains Saturday night. Convective capable of
producing excessive rainfall are expected. There was good model
agreement with the evolution of the synoptic scale systems and QPF
among the 12Z guidance, which is a little farther south toward the
instability axis than the 00Z guidance. Therefore, the WPC QPF and
Excessive Rainfall Outlook were based on a multi model blend of
12Z guidance.

Shortwave energy ejects from a long wave trough over western
Canada crosses the Northern Rockies Friday night, with an
associated surface low developing over the central High Plains
Saturday. Ahead of the front and short wave energy, a 20 to 30
knot low level southwest flow transports 1.75 to 2 inch
precipitable water air (which is between two and three standard
deviations above the mean) across northeast WY and western SD by
peak heating Saturday. Expect instability (1500/2000 J/Kg MLCAPE)
and the to feed organized convective development over northeast WY
Saturday afternoon, which tracks into western SD that evening.

Initially, the convection is expected to be mainly outflow and
hail dominated over northeast WY, but there is a window for short
term training just ahead of the short wave axis. Hourly rainfall
totals could exceed 1.25 inches here, where three hour flash flood
guidance values are as low as 1.50 inches. As the convection
crosses western SD, there is a window for short term training and
possible cell mergers before the front moves over the region.
There is a multi model signal for local 2.00/2.50 inch rainfall
amounts across southwest SD into northern NE. Three hour flash
guidance values are as low as 1.00/1.50 inches (north of the NE
Sandhills), as 7 day rainfall amounts across western SD has been
300+ percent of normal.

The Slight Risk was shifted a bit south over northeast WY across
much of western and central SD to the NE border. The convection
moves east overnight with the threat of flash flood continuing.
Given timing differences, the Marginal Risk still covers eastern
SD.


...Central and Southern Rockies/Intermountain West...
Moisture from the Central Plains and instability over much of the
terrain and plains of CO and NM combine to produce storms with
heavy to excessive rainfall Saturday. There was generally good
model agreement with respect to the synoptic setup and placement
of the highest rainfall amounts, so the WPC QPF and Excessive
Rainfall Outlook were based on a multi model blend of 12Z guidance.

A low level east to southeast continues to transports 1.00/1.25
inch precipitable water air across the plains of CO and NM into
the terrain from the Front Range in CO into the Sangre de Cristo
Mountains of CO into northern NM. Under the mid level ridge,
instability reaches 1000/1500 J/KG along the terrain, and model
soundings show the cap breaking in the afternoon. After the
convection develops, it shifts slowly southward along the terrain
into the plains, as the mid-level flow is fairly weak under the
ridge.

Slow moving storms along the terrain fosters an environment
favoring cell mergers and short term training, which in turn
results in hourly rainfall totals in excess of an inch. Seven day
rainfall totals over portions of eastern CO have been 300+ percent
of normal, with three hour flash flood guidance values as low as
1.00/1.50 inches. Given the moist antecedent conditions, and the
potential for high rainfall rates with slow moving cells, the
Slight Risk was maintained over portions of eastern NM into
central NM.

Hayes/Jackson

Day 3

The Day 3 outlook will be updated by 2030Z.


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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