Area Forecast Discussion
Issued by NWS Des Moines, IA

Home | Current Version | Previous Version | Graphics & Text | Print | Product List | Glossary On
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50
000
FXUS63 KDMX 262332
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
632 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 SPC 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 LLJ 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
concerns.

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
rounds.

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 00Z TAFS through 00Z Tuesday evening/
Issued at 632 PM CDT Mon Jun 26 2017

Weak trough axis will push south of the state tonight with weak
pressure gradient across the state. Winds will become light and
variable overnight with widespread VFR conditions. Some patchy fog
will be a possibility given the rainfall earlier today but the
airmass is quite dry and should limit overall extent. Winds will
increase from the south through Tuesday as warm advection returns.
Widespread VFR conditions will persist during this time.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...NONE.

&&

$$

SHORT TERM...Ansorge
LONG TERM...Curtis
AVIATION...Cogil


USA.gov is the U.S. government's official web portal to all federal, state and local government web resources and services.