Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
841 PM EDT Sat Jun 06 2020
Valid 01Z Sun Jun 07 2020 - 12Z Sun Jun 07 2020
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
NORTHERN AND CENTRAL FLORIDA...
...Gulf Coast into northern-central Florida...
The continued influx of moisture well north-northeast of Cristobal
within its primary inflow band, along with broad, subtle low-mid
layer deformation and frontogenetical forcing across the
central-eastern Gulf Coast south of the upper level westerlies,
will allow for highly-efficient outer rainfall bands along these
coasts and especially into northern FL and portions of the
Panhandle. The dense cloud shield and resultant limited deep-layer
instability (MUCAPEs likely capped around 500 mb) would be a
limiting factor for intense hourly rainfall rates across the
interior Southeast, though enough MU CAPE is expected to persist
across the FL peninsula to continue heavy rain-related issues. PW
values between 2.25-2.50" (2.5 to 3.0 standard deviations above
normal) along with the synoptic scale forcing and higher
instability across the FL Peninsula will allow for widespread
activity and rain rates up to 3" an hour in heavier
showers/thunderstorms, as supported by the latest HREF 40km
neighborhood probabilities. Areal average totals within the Slight
Risk region are expected to range between 2-3+ inches, though per
the high-res CAMs, localized totals of 5-7"+ are anticipated.
...Northern Plains into the Upper Mississippi Valley...
Models remain in good agreement that a shortwave trough will be
riding along the nose of an intensifying LLJ and excessive BL
theta-e ridge (>350K) will drive upscaling convection with heavy
rainfall. Recent activity has been building across central MN
with more isolated storms farther west. The ECMWF and its
ensembles remained generally drier than the NCEP guidance, likely
owing to subtly warmer 700mb temps of 12-14C which could serve as
a lid to convection in addition to its normally dry bias. However,
the synoptics and thermodynamics otherwise support a round of
thunderstorms with heavy rainfall. The best overlap of moisture
and instability lies across SD into eastern ND and northern MN,
where the heaviest QPF is expected. 00-12Z HREF probabilities of 3
hourly rainfall rates exceeding FFG peak between 20-50% over these
areas between 00-12Z. This region has been quite dry recently with
14-day rainfall anomalies of generally less than 50% leading to
higher FFG. Storm motions could be quick as noted by 0-6km mean
winds approaching 50 kts. However, excessive rain rates and
training potential still pose a low-end FF risk across this region.
...Northern and Central Rockies...
Extremely anomalous PW anomalies rising between 3 and 4 standard
deviations will lift northward across ID, MT, northern UT into
western WY in advance of a shortwave transiting UT at this time.
PWs of 0.75 inches to locally 1" will spread rapidly northward,
with height falls/PVA and increasing jet-level diffluence driving
ascent. A couple rounds of thunderstorms have occurred and
another is possible, so despite what could be rapid motion,
repeated heavy rainfall could pose a flash flood risk as FFG is
generally 1.5-2"/3hrs. The area has recently been dry, but
rainfall rates of greater than 0.5"/hr are likely, which could
pose a flash flood risk, especially within any terrain, slot
canyons, or arroyos.
Valid 12Z Sun Jun 07 2020 - 12Z Mon Jun 08 2020
...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL EMBEDDED WITHIN
BROADER SLIGHT RISK AREA THAT COVERS PORTIONS OF THE CENTRAL AND
EASTERN GULF COAST IN ASSOCIATION WITH CRISTOBAL...
20Z update... Slight Risk area was extended further into northern
Florida where some stronger convective bands will likely pass in
connection with Cristobal; as well as, further west and north over
Louisiana and Mississippi. The Moderate Risk area was expanded to
cover more of southern Mississippi and eastern Louisiana where a
broad area of 3 to 6+ inches with isolated higher amounts now
forecast. These changes were coordinated with the local forecast
offices in the Lower Mississippi Valley.
Previous Discussion... Introduced a small Moderate Risk area over
a portion of far eastern Louisiana into southern Alabama as
moisture streams northward and the dynamics increase as Cristobal
approaches on Sunday. While not in absolute agreement in terms of
placement, timing or amounts, there was a broad agreement in the
models that this part of the Gulf would have the best period of
moisture inflow and the best potential to be impacted by multiple
rain bands given the latest NHC track forecast.
The forecast track from the National Hurricane Center was pretty
close to the earlier forecasts which resulted in fairly minor
adjustments in terms of timing and placement on Sunday and Sunday
evening. This will place the axis of heaviest rainfall amounts
across eastern LA/southern MS, attendant with the strong moisture
transport east of the low and favored region for moisture flux
convergence. The enhanced moisture convergence as Cristobal moves
inland would suggest continual heavy rain will move into the parts
of Louisiana and Alabama that had received several rounds of heavy
rainfall earlier in the week. The quicker speed in Cristobals
motion noted on Friday continued and the associated rainfall
forecast was nudged a bit farther north than in the previous
forecast. Highest rainfall amounts of 3-6 inches remain over
eastern LA/southern MS...and 2 to 4 inches now extends northward
into Mississippi as well. Farther east, there was no change to
the Slight Risk area into Florida with the potential for brief
heavy to excessive rainfall rates linger throughout the day. Any
Maintained the Marginal Risk area across the Northern Rockies.
Forcing has been trending weaker, but there will be potential for
rain to exacerbate snow melt. That would lead to a locally higher
risk for flash flooding.
There appears to be support for back building ans training into
the eastern Dakotas. With higher QPF in the WPC forecast and
dynamic support, expanded the Marginal Risk area to the southwest
further into South Dakota.
Previous Discussion... A trough moving into the west coast will
eject a series of shortwaves across the Northern Plains into the
Upper Midwest through the period. The first shortwave will be
exiting to the north and east into Ontario to start the period.
Thus, expecting ongoing showers/thunderstorms Sunday morning.
After this first impulse moves out of the region, expect a short
reprieve during the afternoon with instability (MUCAPE climbing
over 2000 J/kg) rebounding thanks to diurnal heating. With
precipitable water values over 1.75 inches aided by strong (>45kt)
low level southwesterly flow, expect the energy from the second
shortwave to be enough to initiate convection overnight with the
strengthening 850mb low level jet. Spaghetti plots depict the
potential for some 1 to 3 inch combined rainfall amounts in this
region given multiple rounds of showers/thunderstorms during the
Day 1 and Day 2 periods. Soils may become vulnerable to
additional rainfall. Therefore, saw no reason to make many
changes to the on-going Marginal Risk area for this region.
Valid 12Z Mon Jun 08 2020 - 12Z Tue Jun 09 2020
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM THE CENTRAL
GULF COAST STATES NORTHWARD TO THE MID MISSISSIPPI VALLEY IN
ASSOCIATION WITH CRISTOBAL...
...Central Gulf States Northward to the Mid-Mississippi Valley...
20Z update... As mentioned in the Day 2 discussion, there will be
strong dynamic support for slow moving, efficient rain producing
convective across the Northern Plains and into the Upper
Mississippi Valley. With training, backbuilding and influx of
deep, tropical moisture several locations could see 1 to 3 inches
of rainfall over areas that have had recent rains/sensitive to
additional rainfall. There will be an elevated risk for flooding
concerns. Per coordination with the local forecast offices in that
region, felt at this time it was prudent to hold off on upgrading
to a Slight Risk but may need to revisit in the next forecast
Previous Discussion... Cristobal will be making its way northward
across Louisiana into Arkansas and Missouri during the period.
Given the deep layer shear, the bulk of the precipitation should
remain on the east side of the cyclone track especially once
Critobals northward motion become more northeasterly with time.
Issued a Slight Risk area where model QPF tended in the 2 to 4
inch range was coincident with Flash Flood Guidance that had been
lowered to 2 inches or so by recent bouts of heavy rainfall. In
coordination with the affected offices, no Moderate Risk area was
issued at this time although one could be issued later if the
signals become better defined. The fact that Cristobal should be
accelerating during the period was also a consideration not
issuing any category greater than a Slight Risk.
...Upper Midwest to Parts of the Missouri Valley...
With Cristobal inland during this period and a trough/cold front
approaching, the axis of precipitation is nearly south-to-north
over the Lower Mississippi Valley and Ozarks regions. The latest
WPC QPF has a broad 3+ inches area over northern
Louisiana/Mississippi to the Arkansas/Missouri border. With heavy
rain the previous day combined with several inches during this
period, the risk for flooding will increase significantly. The
Moderate Risk was closely collaborated with the local forecast
offices across the Lower Mississippi Valley.
Previous discussions... A mid-level shortwave trough will be
advancing eastward across the Great Basin towards the High Plains.
This will help push a surface cold front south and eastward during
the day...and the front will provide a focus for late day storms
from Minnesota to the Missouri Valley. Locally heavy rainfall
rates and amounts are expected given the amount of instability
available and precipitable water values running at or slightly
above 1.5 inches already in place. Even so, storms should be
progressive...which should limit the extent of flash flooding.
The expectation is that the deep moisture associated with
Cristobal will remain east of this system. If this system taps
some of the moisture-rich environment from Cristobal, rainfall
amounts and rainfall rates could be enhanced with heightened risk
of excessive rainfall.
Day 1 threat area: www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Day 2 threat area: www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Day 3 threat area: www.wpc.ncep.noaa.gov/qpf/99epoints.txt