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Area forecast discussion
National Weather Service Des Moines Iowa
350 PM CDT Mon Jun 26 2017

Short term.../tonight through Tuesday/
issued at 347 PM CDT Mon Jun 26 2017

20z water vapor imagery showed moisture associated with the showers
earlier in the day over northeast Missouri into central parts of
Illinois and Indiana. A weak pv anomaly was over siouxland that will
dive across the state the remainder of the afternoon into the
evening. At the surface, a weak trough was draped from west to east
over southern Iowa. Northerly winds and dewpoints in the 40s over
northern Iowa were found north of trough with southeasterly winds
and dewpoints in the 60s south of the trough. 20z Storm Prediction Center mesoanalysis
showed weak moisture convergence and between 500 and 1000 j/kg of
cape, which should help to form a few isolated showers or storms
over parts of southern Iowa. These will push into Missouri this
evening as the pv anomaly moves away and surface high pressure
moves into the state. Winds will become light and variable and a
few patches of fog may form over the southeast part of central
Iowa as high pressure traverses the state overnight. The high will
continue to slide off to the southeast and winds will become from
the south tomorrow. As a warm front approaches, this will help to
tighten the pressure gradient with winds becoming breezy,
especially over western Iowa. Clouds will also begin to increase
later in the afternoon as well.

Long term.../Tuesday night through Monday/
issued at 347 PM CDT Mon Jun 26 2017

Summary...high temps to reside in upper 70s - mid 80s generally
throughout forecast, with increased low temps into the 60s as
moisture returns to area beginning Tue. Easily main concern through
this time period is severe weather and heavy rainfall/flooding
potential Tue night through Fri.

Tuesday night through Friday...area will be on backside of departing
surface high with return flow well under way by Tue
afternoon/evening. With low to mid levels very dry, shower/storm
initiation will be slow even with good Theta-E advection and
probably not until low level jet initiates convection over parts of Nebraska.
That complex of storms will move eastward overnight and be working
with limited instability, under 1500 j/kg, but shear around 35-40
kts could provide some organization and boost to hail and wind
threats. The bigger threat though will be Wed afternoon/evening as
high instability works back in over western Iowa upwards of 4000+
j/kg SBCAPE along with ~40 kts 0-6 km shear. Certainly raises
concern for organized storm development with very large hail and
damaging wind potential. Tie in low level support such as 0-1 km
shear and helicity of 20-25 kts and ~200 m2/s2 respectively and LCLs
under 1000m and a tornadic threat certainly exists. Somewhat of a
wild card at the moment will be how morning convection plays out and
what kind of cap ultimately is in place over the area. A lesser cap
will lead to messier/widespread convection and likely less high end
severe potential versus a bit stronger cap that may allow periods of
discrete or small clusters of storms. Regardless of ultimate Mode
and severe potential, heavy rainfall threat will exists with pwats
in excess of 1.75 inches. However only a somewhat limited
flooding threat due to antecedent dry conditions the last couple
days. Given strong, consistent signal, have boosted pops into
categorical range in the Tue overnight time frame, but have held
off boosting into the afternoon due to previously pointed out

Thursday will also pose another storm threat as a weak boundary
extending off surface low north of the state likely stalls out
across southern Iowa. While location may be in question a bit,
severe threat will be there south of the boundary with likely
3000+ j/kg MUCAPE and 35+ kts of 0-6km shear will be present.
Should southern Iowa be in line again, flooding threat will
increase with likely already saturated grounds from previous two

Lastly within this window, as shortwave trough pivots through
another round of storms will be possible. However, details at this
point highly suspect given multiple rounds of convection between now
and then. That said, severe potential may once again be in play with
some depictions of high cape but borderline supportive deep shear.
As hinted for Thursday round, flooding concern may peak here should
similar areas be slammed multiple times between now and this point.

Saturday through Monday...Canadian upper low will move into northern
plains and pivot through. Will bring about another opportunity for
storms, though sub severe, as shortwaves move through the parent
flow and temperatures remain on the mild side. By Monday, ridging
looks to being to move back in with storms on the heels of another
wave of Theta-E advection.


Aviation.../for the 18z tafs through 18z Tuesday afternoon/
issued at 1234 PM CDT Mon Jun 26 2017

Morning showers and widespread clouds have pushed south of the
state while scattered cumulus have developed in their wake. Per
observations, the lowest clouds bases range from 2000 to 3000
feet at a few places. With these clouds progressing through any
given site in an hour or so, have gone ahead with VFR conditions
through the taf period. There may be periodic, brief periods of
MVFR ceilings, but not enough confidence in timing the impact at
the terminals. By this evening, clouds will clear and winds will
become light as high pressure passes through the region.


Dmx watches/warnings/advisories...


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