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FOUS30 KWBC 261733
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
132 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 isn't 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 become focused along and ahead of a
frontal boundary crossing northeast WY into SD, which supports
convection capable of producing excessive rainfall. There was
generally good model agreement concerning the evolution of the
synoptic scale systems (though the 00z GFS may be too weak with
the front over SD), so the WPC QPF and Excessive Rainfall Outlook
were based on a multi model blend.

Short wave energy ejected from a long wave trough over western
Canada crosses the Northern Rockies during Day 2, pushing a
frontal boundary over the Northern Rockies into the Northern
Plains, mainly during the second half of Day 2. Ahead of the front
and short wave energy, a 20/30 knot low level southwest flow
transports 1.75 inch precipitable water air (which is between two
and three standard deviations above the mean) across northeast WY
and western SD, generally between 27/18z and 27/00z. Model
soundings showed 1500/2000 J/KG of MLCAPE over this area, and the
combination of moisture and instability are expected to feed
convection over northeast WY before 28/00z, which tracks into
western SD between 28/00z and 28/06z.

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. Hourly rainfall rates
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 western into central SD. Three hour flash guidance
values are as low as 1.00/1.50 inches, as 7 day rainfall amounts
across western SD has been 300+ percent of normal.

Based on the above, a Slight Risk was placed over northeast WY
across much of western and central SD. As the convection moves
east late in the period, the threat of flash flood continues, but
the storms could be more progressive over eastern SD before
28/12z.


...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 during Day 2. 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.

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 27/18z to 28/00z time
frame. After the convection develops, it trundles 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 rates in excess of an inch (per the 00z
NAM CONUS Nest). 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, a Slight Risk was placed over portions of
eastern NM into central NM for Day 2.

Hayes

Day 3
Valid 12Z Sun Jul 28 2019 - 12Z Mon Jul 29 2019

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER THE UPPER
MISSISSIPPI VALLEY INTO THE UPPER GREAT LAKES...

...Upper Mississippi Valley into the Upper Great Lakes...
Deepening moisture and strong instability ahead of short wave and
cold front crossing the Northern Plains into the Upper MS Valley
and Upper Great Lakes feeds convection capable of producing
excessive rainfall during Day 3. There was generally good model
agreement concerning the overall synoptic setup, so the WPC QPF
and Excessive Rainfall Outlook were based on a multi model blend.

Convection is expected to be ongoing (and weakening) as it tracks
from SD into MN during the first part of Day 3. Debris cloudiness
from this activity could delay heating across MN into WI during
the morning hours. However, ahead of the front and mid level
trough approach from Manitoba/MT, instability climbs to 2500/3550
J/KG of MLCAPE stretching from eastern NE/IN across southern MN
into WI and the northern portion of the LP of MI, peaking between
29/00z and 29/06z.

The developing convection is feed 2.00+ inch precipitable water
air (which is about two standard deviations above the mean) on a
30/35 knot low level southwest flow during this time frame. Lift
ahead of the long wave trough should allow the convection to
expand in coverage, and become better organized as it moves over
southern and central MN into northern and central WI. This airmass
could support hourly rainfall rates near 1.50 inches, and there is
a multi model signal for 2.00/2.50 inches of rainfall over central
MN into northern WI, closest to the best instability gradient.

Three hour flash flood guidance values are as low as 1.50 inches
over portions of MN into WI (including the Minneapolis metro
area), and these numbers could be exceeded where convection trains
ahead of the front. Based on this, a Slight Risk was placed over
much of southern and central MN into northern WI. The placement of
the Slight Risk could vary in later forecasts, especially if the
instability trends south with time.


...New Mexico...
While the mid level ridge eases to the west, and the monsoon flow
becomes suppressed temporarily, deeper moisture and instability
across eastern NM could support convection with heavy to locally
excessive rainfall during Day 3. Storms developing in an axis of
1000 J/KG of MLCAPE extending from KTCC to KSRR are expected to
tap 1.25 inch precipitable water air (which is about two standard
deviations above the mean) in place over eastern NM to support
convection capable of producing hourly rainfall rates near an inch
along and just west of the terrain (as the storms moving slowly to
the west with the mid level flow).

Three hour flash flood guidance is generally near 1.50 inches, and
it is possible that these amounts could be realized where storms
merge or where short term training occurs. Since there is some
spread in coverage of storms during Day 3 (as instability could be
a limiting factor for a more organized flash flood threat), a
Marginal Risk was placed over a portion of east central NM.

Hayes


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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