Scientific Forecaster Discussion
fxus63 kdmx 181728
Area forecast discussion
National Weather Service Des Moines Iowa
1228 PM CDT Wed Sep 18 2019
Discussion.../today through Tuesday/
issued at 431 am CDT Wed Sep 18 2019
-several rounds of showers and thunderstorms likely over the next
-Heaviest rainfall may be in central to northern Iowa, with a non-
zero chance of flooding issues starting tonight/tomorrow and
-Above-normal temperatures to continue...
Another seasonably hot and humid day is in store for central
Iowa. Expect temperatures for today to be similar... perhaps a
degree or two cooler than where they were yesterday, topping out
well into the 80s. With dewpoints expected to remain in the low
70s, heat index values will likely push the low 90s once again in
central to southern Iowa.
For tonight, confidence is high in heavy to moderate rainfall
occurring across north-central into south-central Iowa. These
showers and storms are expected to start in northern Iowa and very
slowly move southward through the overnight hours. Overall storm
motion may be less than 5 mph. In total, with this prolonged
rainfall event stretched into Thursday afternoon, rainfall amounts
in some locations may reach the 1 to 2 inch range. Localized flash
flooding cannot be ruled out. Today's forecasters will look closely
at the need for any type of flood headlines for tonight/tomorrow.
Yet another slug of showers and storms are forecast sometime near
Saturday that could bring widespread moderate rainfall totals.
Forecast confidence is somewhat low at this time as there is a large
amount of spread in terms of model timing and location of the
heaviest precip chances.
Overall, Sunday will be slightly cooler, but there is no
strong/prolonged cold air advection in sight behind the Saturday/Sunday
system...meaning next week's temperatures will likely remain in the
70s to 80s. If any sustained sunshine enters the forecast, next
week's temperatures will need to be bumped up at least 5 degrees.
*/ Technical discussion...
08z water vapor imagery clearly picking up on an upper low spinning
over northwest North Dakota. Well-phased in the positive vorticity advection side of this upper low,
a weaker impulse is being propagated east/NE across Minnesota.
Aloft, decent 500mb-300mb q-vector convergence that is leading a
broad swath of 700mb qg frontogenetical forcing is in play over
northwest Iowa... supporting ascent processes. Lower in the atmos, a 40kt
low level jet has transported a huge slug of moisture that is advecting
pwats of nearly 2 inches into northwest Iowa, focused with a broad area
of 850mb moisture flux convergence. In response, showers and
thunderstorms have developed across the northwest dmx County Warning Area. Rainfall
amounts of 0.5 inches or so are currently forecast...with a few
localized amounts nearing 1 inch certainly possible. Hi-res models
in good agreement with gradually propagating this mesoscale setup
eastward through today, with precip slowly diminishing as it
nears the eastern edge of the dmx County Warning Area by the afternoon.
Another shortwave blasts through the upper Midwest between 06 and
12z Thursday. Similar to this morning's setup, abundant moisture
in play as precipitable water values near 1.9 inches... which is an upwards
trend from last night. Heavy rainfall parameters favorable and
indicate moderately efficient rain producers. Corfidi vectors
effectively become nil and overall storm motions slow greatly. So
this system looks to play out as a giant swath of rain that starts
in north- central Iowa this evening, and very slowly drifts
southeastward through the overnight hours. Seemingly, the rap has
been the most excited about rainfall totals as it has been
yielding amounts in the 1 to 3 inch range for this timeframe.
As mentioned yesterday, the north-central portion of Iowa... from
Ames to Fort Dodge to Mason City... has received roughly upwards of
600 percent of their normal rainfall totals over the past 7-10 days.
So antecedent soil conditions are very saturated. Further, will
have to see how this morning's rainfall plays out across the area.
While no notable flooding is expected from this morning's
rainfall, it may saturate the grounds in advance of tonight's
system. The possible need of flood-related headlines will be
looked at closely by today's forecast shift. Given the efficiency
of the rain producers from this morning, am becoming increasingly
nervous for tonight.
A rather potent-looking upper low is being progged to come onshore
near the Oregon coastline and dip down through The Four Corners
region by Friday. Meanwhile, another shortwave is being progged to
propagate eastward across the northern Continental U.S.. these two features
more-or-less fujiwara over the upper plains on Friday and eject
towards Minnesota Saturday. Further, the moisture associated with
the tropical system, Imelda, may be phased into these features as
well...being advected directly over Iowa sometime Saturday/Sunday.
Right now, long-range models do not have the best handle on how
these interactions will all play out. The GFS has been faster than
the European model (ecmwf)... and the 00z Wed GFS has catered more to the European model (ecmwf)
solution. So, have slowed precip timing slightly since yesterday
evening... mentioning the potential for this to carry into Sunday.
Long-range models are pushing pwats of 2 inches into Iowa with this
setup. So will have to monitor this for another moderate to heavy
rainfall potential. Given what rain falls Thursday, this could lead
to "compounded" flood-related impacts. Due to high model spread,
confidence too low at this time to get too mired in the details of
timing/location/amounts. But will be watching this closely.
Aviation.../for the 18z tafs through 18z Thursday afternoon/
issued at 1228 PM CDT Wed Sep 18 2019
Line of storms was previously expected to diminish continues to
work its way across central Iowa and could impact Kalo and kdsm
through early afternoon. Otherwise it is expected to dissipate
before reaching kotm. A surface boundary is situated across
northwest Iowa and is expected to be the focus for further
storm/shower development later this evening and continuing through
the taf forecast period. This boundary is expected to stall its
progression across central Iowa which would allow for an extended
period of rain showers/thunderstorms and rain impacting Kalo and kdsm Thursday morning. For
now have highlighted the greatest chances for thunderstorms and rain/rain showers at all taf