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Area forecast discussion 
National Weather Service Des Moines Iowa
323 PM CDT Thursday may 28 2015

Short term /tonight/...
issued at 323 PM CDT Thursday may 28 2015 complicated forecast tonight
as we have a few things affecting the forecast. Adding to the
mix...the models and in particular the hi- res models do not seem
to have a great handle on the Mode of convection or the coverage
of it.

Remnants of the mesoscale convective complex over MO from last night initially were dying out
as the showers/storms lifted to the north and ran into the surface
high over Iowa. The high however is shifting to the east and return
flow/weak warm advection has shifted into southwest Iowa. This
allowed for re-generation of showers across southwest Iowa. Also a
shortwave lifting out of Kansas is currently pushing into southwest
Iowa and aiding in the re-generation of showers. It will lift
across the forecast area this evening into the overnight...reaching
northern/northeast Iowa between 04z and 07z.

Thus far instability is weak and there is very little to no shear so
what we have over southern/southwest Iowa is showers with a
potential for an isolated thunderstorm through late afternoon. The
shortwave should provide enough forcing and coupled with some Theta-
east advection...thunderstorms should become at least a little more
prevalent through the evening. Having said that though...with such
meager instability and weak to non-existent shear...the coverage of
thunderstorms is really in question.

We may see more of a showers with isolated thunderstorms type
scenario through the event this evening. Given the weak shear over
the area even if we do get a thunderstorm...the threat of any storm
being severe is minimal. However with a deep warm cloud depth...
locally heavy rainfall is possible from any stronger storms.

As for coverage...the hi-res models are having a horrible time with
how the precipitation will evolve across the County Warning Area and move across. Based on
current radar trends...the precipitation shield should continue to expand
across SW Iowa and lift to the north northeast through the evening
with most of the precipitation being done by 06z so. There should only be
one event tonight with the shortwave passing but all of the hi-res
models linger some spotty showers/storms across the County Warning Area throughout
the night likely from lingering qg forcing and Theta-E advection and
while that doesn't seem likely...it is difficult to pinpoint where
lingering precipitation might be and where it will be absent. I hit the
probability of precipitation highest during periods when the shortwave will lift across then
drop the probability of precipitation significantly for the rest of the period. This may be
overdone but keeping high chance or likely probability of precipitation going for a 12 hour
period would also be overdone. Later shifts can adjust as this
event evolves across central Iowa. The 18z NAM actually has finally
picked up on this though it is close to publish time. The forecast
is at least trended in that direction. Probability of precipitation should again pick up
towards/after 12z as another short wave approaches central Iowa.

Also there may be some patchy light fog developing in areas that
have a little better rainfall from the evening but this should not
have an impact to central Iowa commuters.



Long term /Friday through Thursday/...
issued at 323 PM CDT Thursday may 28 2015

Pattern continues to be active through Friday night...with the main
upper level system approaching the region Friday. This will bring
more showers/thunderstorms before cooler high pressure builds in for
the weekend. Current water vapor imagery shows multiple waves over
the Southern Plains. Along with the mean trough advancing
southeast...a short wave currently over northern New Mexico will
drift northeast with time and arrive over southern Iowa by 21-00z
Friday. The combination of the approaching cool front and shortwave
Friday will increase the chances for storms especially in the
afternoon/evening hours. Overnight temperatures will remain mild...with
highs Friday cooler than today...with clouds and showers more
likely. Widespread severe chances again appear lessened Friday...
as models show best bulk shear arriving behind the boundary Friday
evening. Meanwhile...MUCAPE should increase to 800 to 1600 j/kg
between 18 and 00z Saturday. With the lack of any cap...likely
that gradual release of instability will become more common in the
afternoon hours as the mornings convective system diminishes in
intensity. Biggest threats later in the day should be from wind
and hail as forecast upper level wind fields remain marginal and
should reduce storm organization. With the airmass in place
now...precipitable water values will continue to increase as mass
convergence increases through Friday afternoon ahead of the
boundary. Precipitable water values approach 1.5 to 1.75 from i35 east southeast
during the afternoon hours and early evening. Concurrent warm
cloud depths will top out at 10 to 12kft for most of the southeast
two thirds of the forecast area. Despite the reality of efficient
rainfall...most of the gridded flash flood guidance suggests 1 and
3 hour values near 2 inches... and 6 hour near 2.5 inches. The
overall threat for flash flooding remains low...however bufr
soundings indicate that there will be a 2 to 3 hour period of slow
moving storms possible over the area during the day through
evening. Over the northwest between 10 am and noon...the Highway
20 corridor in the early afternoon...the i80 corridor in the early
evening. Though there is some uncertainty about overall coverage...
there is a chance for some urban run off issues if a quick 1 to 2
inches of rainfall falls in some of the heavier storms. The
chances for additional locally heavy rainfall should shift to
southeast Iowa between 00 and 06z as the boundary progresses
southeast and the diurnal increase in h850 winds arrives over that
area between 03-06z. Once the storms push southeast rainfall
should diminish into 12z Saturday morning with cooler and less
humid conditions for the balance of the day Saturday. Over the
weekend h850 temperatures will cool to 6 to 8c over the region...this
will cap highs in the upper 60s to middle 70s with comfortable
humidity.

In the long range the mean h500 pattern remains active with a relatively
persistent subtropical ridge over the southeast US and a rather
cool polar jet across the Pacific. The combination of these two
features is likely to promote additional shortwaves to Peel off
the main flow and develop into The Rockies/plains while h850 flow
remains directed at the plains states through the extended period.
The next wave to affect the area should arrive around Wednesday
night of next week with chances for storms lingering into the end
of the extended. High temperatures through the period will remain
warm with middle 70s to middle 80s most likely and lows in the 60s.

&&

Aviation...28/18z
issued at 104 PM CDT Thursday may 28 2015

Complex forecast as there will be several rounds of showers/storms
affecting taf locations. Remnant showers have lifted out of MO and
additional development will be aided by a shortwave lifting out of
the SW moving across the forecast area. As the shortwave moves
across it should provide enough forcing for thunderstorms to
develop. There will then be a break late tonight/Friday morning
before the cold front moves through and additional storms form. A S
to southeast surface flow will be common through the forecast period at 15-25kts and
diminish after sunset. Local MVFR conditions will be possible in
showers and possibly some fog late tonight.



&&

Dmx watches/warnings/advisories...
none.
&&

$$

Short term...fab
long term...rev
aviation...fab

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