FXUS63 KDVN 190914
Area Forecast Discussion
National Weather Service Quad Cities IA IL
414 AM CDT Thu Sep 19 2019
Issued at 328 AM CDT Thu Sep 19 2019
Strong ridging continuing to be in place across the central part of
the country. Overnight a weak shortwave trough moved over the top of
the ridge and that along with a decent low level jet and CAPE values
of 2K to 3K, thunderstorms have developed from eastern Nebraska into
northeast Wisconsin. While the primary band of thunderstorms is
north of the local area, a good feed of moisture has being observed
at 925mb directed into the northwest portions of the forecast area
through the night which has kept scattered showers and thunderstorms
ongoing through the night hours.
Elsewhere, Tropical Depression Imelda has moved onshore over far
southeast Texas. This storm continues to have high amounts of
moisture associated with it, and continues to produce very heavy
rains as it moves inland.
.SHORT TERM...(Today and Tonight)
ISSUED AT 328 AM CDT Thu Sep 19 2019
925mb moisture transport field seems to be the best indicator of the
current convection across the local area, while the more widespread
storms to the north are being fed by moisture at 850mb. While the
flow at 925 mb does have strongest convergence into the
thunderstorms over western Iowa, it does have an arm that extends
into eastern Iowa, which has been veering over the past few hours.
This same trend is being seen in the 850mb low level jet. This would
explain the recent strengthening of the storms across the northern
parts of the CWA as well as the activity to the north. RAP data
indicates that the flow in these levels will continue to veer over
the next few hours and weaken, so thinking that coverage of the
isolated to scattered showers and thunderstorms over the local area
will wane. However, models indicate that the shortwave trough in the
mid-levels will sink southward a bit, which will then allow the line
of storms to the north to also sink southward into the forecast
area. Timing would put these storms into the northern parts of the
CWA around 11-12Z. The MCS then could either follow the upper level
support and progress eastward, waning in strength as it moves east
or it could then continue to progress southeast into the CAPE axis.
The low level moisture feed deteriorates through the morning, and
the instability is substantially less going eastward so there is
considerable uncertainty as to how far the MCS will make it before
dissipating. Ultimately, the best chances for thunderstorms will be
across the northern parts of the forecast area, with much less
confidence in any activity south of Highway 30. Precipitable water
is currently around 1.70 inches, and the freezing level on the DVN
19.00Z sounding at 14,400ft, the warm cloud depth is quite deep so
storms will be efficient rainfall producers so any storm will have
the potential to produce torrential rainfall. But, with the
uncertainty as to the coverage of storms locally, think any flash
flood threat will be more localized than widespread so at the
current time did not opt to expand the Flash Flood Watch that is
currently in place to our north.
The surface boundary stays in place across the area through tonight,
and with the models showing weak shortwave energy moving over the
ridge throughout the period, there is chances for storms throughout
the period. Did attempt to time in the higher chances for wet
periods, with this evening being the period with the best chances
for being dry.
Did also lower temperatures today as much of the day will be cloudy
with periods of rain. It'll likely still be above normal and humid,
but highs will be lower than what was experienced yesterday.
.LONG TERM...(Friday through Wednesday)
ISSUED AT 328 AM CDT Thu Sep 19 2019
Low-level winds will gradually respond to low pressure over the
High Plains, veering from the east early in the morning to an ESE
or southerly direction later in the day. Building instability
into the 1500-3000 J/kg range, a NW to SE 925-850mb convergent
axis through the central to northern CWA, and period of weak
isentropic lift could kick of scattered showers and storms. Any
storms during the morning would likely be elevated, then have a
better chance to become surface-based during the afternoon as
temps climb into the mid 80s on average. Overall coverage should
be low (20-30%) and thinking the best chances are along and north
of highway 30. Locations that are not impacted by storms should
top out in the upper 80s to lower 90s for afternoon peak heat
The main message continues to be a strong signal for showers and
thunderstorms and a threat for bouts of heavy rain as a mid-level
trough interacts with an anomalously moist atmosphere emanating
from the Western Gulf (and moisture associated with the remnants
of a tropical low over NE Texas). The NAEFS has integrated WV
transport values near 3 standard deviations above the mean and
925-850mb mixing ratios close to a 1 in 30 year return interval
for this time of year.
Models are in decent agreement on QPF potential with this event,
the core of which will fall from Saturday evening into Sunday,
with a widespread area receiving 1-3 inches. The exact placement
could still shift north or south, but latest model consensus puts
the heaviest swath in the counties along and south of I-80.
Very high PWATs (~2"), fueled by a strong atmospheric river with
connections to the Gulf and even western Caribbean, could support
high rain totals over 3 inches and lead to potential flash
flooding in the hardest hit locations. However, it is too early
to narrow down the most likely 1-2 county wind corridor that will
receive these higher end amounts.
Please stay tuned to the forecast over the next several days,
especially if you have interests on area rivers or live in a
location that is susceptible to areal flooding or flash flooding.
If this forecast remains on track, flash flood watches may
eventually be needed. In addition to the risk for heavy rain, the
Storm Prediction Center has a slight risk for severe storms in
the west-central forecast area with a marginal risk to the east
(Saturday and Saturday night).
Lower humidity and temps in the 70s return for early next week.
The model blend has low chances for rain by Tuesday and Wednesday.
Confidence is low on the rain potential with the ECMWF completely
dry and the GFS bringing a shortwave through the area. Uttech
.AVIATION...(For the 06Z TAFS through 06Z Thursday Night)
ISSUED AT 1251 AM CDT Thu Sep 19 2019
Showers and thunderstorms have moved into the northwest parts of
the forecast area overnight, and radar shows further development
towards central Iowa. Movement though is to the north-northeast so
there is some uncertainty to how far east it will get tonight and
if any of the TAF sites will be impacted. Did include thunder at
KCID and KDBQ as it seems reasonable that these terminals will
get hit by some activity before the focus returns out west of the
area. With the cold front looks to drop south then through the day
today which will bring thunderstorm chances to KMLI and KBRL this
afternoon. Timing is still highly questionable though and there
is a possibility that at least scattered showers and thunderstorms
could be seen across much of the area into this evening but with
little confidence in that scenario, did not put this in the
forecast at this time.
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