Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
438 PM EDT Tue Jul 31 2018
Day 1
Valid 1818Z Tue Jul 31 2018 - 12Z Wed Aug 01 2018
...A SLIGHT RISK OF EXCESSIVE RAINFALL FROM THE LOWER MISSISSIPPI
VALLEY THROUGH THE SOUTHERN AND CENTRAL APPALACHIANS...
1800 UTC update
An update to the Excessive Rainfall Outlook was based on current
radar trends across several areas.
Over the southeast portion of the L.P. of Michigan...the marginal
risk area was extended northeastward from the previous issuance to
cover heavy rain potential through this evening. The latest
arw---nmmb and nam conest all show heavy rain potential after 0000
UTC in the comma head/deformation precip area to the northwest of
the surface low pushing north northeastward from northern Ohio
into southwest Ontario.
Across the Central Appalachians...the slight risk area was
extended farther to the west into far eastern TN---along the KY/VA
border and into southern WV. This was for enhancing convection
this afternoon ahead of the slow moving front aligned north-south
from the above mentioned low that will be moving northeastward
into Ohio this afternoon. Please see WPC's mesoscale
precipitation discussion #0568 valid until 2330 UTC for additional
information across this region.
The slight risk area was also broadened southern MS into southern
Alabama to cover the enhancing precipitation across these areas.
Please see WPC's mesoscale precipitation discussion #0567 valid
until 2100 utc for additional information across this area.
Oravec
Initial discussion
...Lower MS Valley/Central Gulf Coast states northeastward to the
OH Valley and Appalachians/Mid-Atlantic...
Digging shortwave energy/height falls act to carve out a larger
scale mid-upper level trough and closing low over the mid-lower MS
Valley that will prove slow to progress and reach amplitude
upwards to 3 standard deviations below normal. Deep moisture
channeling northward in between this feature and a downstream
western Atlantic ridge will lead into another prolonged wet period
that was well advertised at medium range time scales. Expect
organized convective rainfall potential to focus both near a wavy
front from the lower MS/TN Valley to the OH Valley with dynamic
support aloft with pooling moisture, with warm sector meso
boundaries, and also with terrain lift through the Mid-Atlantic
Piedmont to Appalachians as high precipitable water air feeds into
the region channeled to the west of the slow moving 2+ standard
deviation from normal western Atlantic ridge. Cell training with
increased LLJ inflow and favorable flow steering aloft is likely
to combine with deep moisture to support a widespread marginal to
embedded slight risk areas of excessive rainfall. Model guidance
is better clustered today compared to yesterday with mass
fields/qpf as supporting larger scale flow comes together,
building forecast confidence. There is a seemingly reasonable
guidance trend to be slower to spread heaviest activity across the
broad region.
...Southwest...
Diurnally driven activity should fire again with differential
heating near favored terrain and then consume available
instability. Fueling moisture is still ample under weak flow aloft
under/around a 1.5+ standard deviation above normal ridge aloft,
so there will be an opportunity for repeat activity and some
locally heavier downpours. Precipitable water values 2+ standard
deviations from normal work more from the desert Southwest into
NV, so the WPC marginal threat area extends from southwest TX
northwestward across NM/AZ and well into NV and into eastern CA.
Schichtel
Day 2
Valid 12Z Wed Aug 01 2018 - 12Z Thu Aug 02 2018
...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ALONG THE
FLORIDA PANHANDLE...
...2030 UTC Update...
Supported by deep southwesterly inflow and favorable upper jet
forcing, there remains a strong model signal for additional heavy
amounts along the Florida Panhandle on Wednesday into early
Thursday. Flash flood guidance values are relatively high across
this region, but may already be on the decline by Wednesday
morning with locally heavy amounts possible across the region
Tuesday evening and overnight. Following coordination with WFOs
Tallahassee and Mobile, a Moderate Risk was added to portions of
the western and central Florida Panhandle, where amounts of 2-4
inches, with locally higher amounts, can be expected.
Elsewhere, with models remaining in generally good agreement with
the larger scale features, made minor adjustments based on the 12
UTC guidance.
...East...
The overall synoptic pattern does not change much across the
eastern third of the country from Day 1 to Day 2. The broad
anomalously strong trough extending from the Great Lakes to the
Gulf Coast will remain nearly stationary as it's trapped between
ridging off the East Coast and over the southwestern states.
Will maintain a Slight risk for portions of the Southeast and
Mid-Atlantic states given continued strong inflow of Gulf
moisture, low-level convergence along the nearly stationary
surface boundary, and persistent upper divergence within the right
entrance region of the jet streak across the OH and TN Valleys.
Overall appears to be a prolonged period where favorable dynamics
and moisture overlap...supportive of a flash flood risk. Main
limiting factor will likely end up being instability...with
extensive cloud cover and poor lapse rates impacting this. With
that said, the strong synoptic forcing should overcome this to
some degree and still allow for a widespread area of moderate to
locally heavy rains. Also would suspect that at least some of the
area is able to destabilize enough to produce some heavier
convective cores and potentially a more focused flash flood
threat. The heaviest rains this period may end up actually being
over the FL Panhandle. This setup of persistent southerly flow off
the Gulf into the coastal front, coupled with right entrance
region jet dynamics, can often result in coastal training
convection. Also, some signal that upslope flow may help enhance
amounts over the southern Appalachians. Depending on antecedent
rainfall, and instability trends, possible that a portion of this
Slight Risk area could be upgraded to a Moderate as we get closer.
...Southwest...
Another day with an isolated flash flood risk from portions of
NV/UT into AZ into NM. Again generally encompassed the area with
overlap of anomalous PWs, CAPE and slower cell motions. Models
suggest there will be more organized activity, particularly across
NM and eastern AZ as short wave energy drops southward down the
front side of the slowly weakening Southwestern U.S. Shear/CAPE
profiles also support more organized convective activity here. PWs
end up only near average by this time, and storms may tend to move
given stronger mean flow. Thus for now will stick with a Marginal,
but continue to monitor for potential upgrade.
Pereira/Chenard
Day 3
Valid 12Z Thu Aug 02 2018 - 12Z Fri Aug 03 2018
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM THE SOUTHEAST
INTO THE MID-ATLANTIC REGION...
...2130 Update...
Based on the model signal for increased precipitation amounts and
coverage, expanded the Marginal Risk further to the north across
Utah, primary along the Wasatch into the Uintas.
Elsewhere, with the models remaining in reasonable agreement and
showing good continuity, made only minor adjustments based on the
12 UTC guidance.
...East...
Pretty much a copy and paste from the day 2 discussion. The
overall synoptic pattern continues to not change much across the
eastern third of the country into Thursday. The broad anomalously
strong trough extending from the Great Lakes to the Gulf Coast
will remain nearly stationary as it's trapped between ridging off
the East Coast and over the southwestern states.
Will maintain a Slight risk for portions of the Southeast and
Mid-Atlantic states given continued strong inflow of Gulf
moisture, low-level convergence along the nearly stationary
surface boundary, and persistent upper divergence within the right
entrance region of the jet streak across the OH and TN Valleys.
Overall appears to be a prolonged period where favorable dynamics
and moisture overlap...supportive of a flash flood risk. Main
limiting factor will likely end up being instability...with
extensive cloud cover and poor lapse rates impacting this. With
that said, the strong synoptic forcing should overcome this to
some degree and still allow for a widespread area of moderate to
locally heavy rains. Also would suspect that at least some of the
area is able to destabilize enough to produce some heavier
convective cores and potentially a more focused flash flood
threat. Some signal that upslope flow may also help enhance
amounts over the southern Appalachians.
Given the multiple day event, gave consideration to a Moderate
Risk on day 3 over portions of northern GA into the western
Carolinas and southwest VA. Still some concerns about instability
and rainfall rates as the event progresses...so in the end decided
to stick with a Slight for now. While a Moderate may very well
eventually be needed, exactly when and where the highest risk
evolves is still uncertain. Thus will wait and see where
antecedent rainfall ends up heaviest and where instability looks
more likely to support higher rates...and consider upgrading based
on these factors.
...Southwest...
Another day with an isolated flash flood risk from portions of
NV/AZ into NM. Again generally encompassed the area with overlap
of anomalous PWs, CAPE and slower cell motions. Overall the
pattern on day 3 appears less conducive for organized convection
than day 2. However, the combination of elevated PWs, instability,
and weak upper diffluence still supports convective development
and localized flash flooding.
Pereira/Chenard
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