Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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FXUS65 KBOU 241815

Area Forecast Discussion
National Weather Service Denver/Boulder CO
1215 PM MDT SUN JUL 24 2016

Issued at 1207 PM MDT Sun Jul 24 2016

Some concerns with the convective forecast for today, chiefly with
the amount of energy that will be available this afternoon. Models
are overdoing the moisture return at least for the afternoon. Dew
points have dropped to around 40 near the Front Range and are in
the mid to upper 50s on the plains even before deep mixing has
begun. Anticipate these numbers to drop a little more this
afternoon before they start climbing again on persistent southeast
winds this evening. This leaves us with a pretty well capped
atmosphere this afternoon. In the mountains temperatures are
similar to the past few days, but it is a little drier there also.
So we should be able to generate mountain storms, but with low
CAPES in and near the mountains until this evening. This calls
into question the severe threat especially near the mountains.
Even when the moisture comes this evening, it may not be warm
enough to generate strong storms. There may be more of a threat
out on the plains where there is richer moisture, but it seems
like the temperature should take the edge off of everything with
CAPEs closer to 1000 J/kg and not the higher numbers in some of
the models. This could be enough for a few severe pulses, and with
a drier environment there could be some wind risk as well, but for
the most part I would expect subsevere storms that are
increasingly wet as they spread eastward. At least with
temperatures in the upper 80s at early evening the hail that does
develop will have a chance of getting to the ground, so isolated
one inch hail does make sense.

All in all little change to our forecast other then tweaking
timing details. I did raise the highs this afternoon along the
Front Range as we are running a few degrees warmer than expected.


.SHORT TERM...(Today and tonight)
Issued at 230 AM MDT Sun Jul 24 2016

A moderate westerly flow aloft will remain over the state
a jet max stretches across northern Colorado...with 250 mb winds of
50-60kts. This has allowed for tstms to persist overnight in the
mountains while it is just now exiting the northeast plains. At the
surface...low level moisture will increase this morning as a cool
front settles along the Front Range Foothills and Palmer Divide. As
a result...temperatures today should be around 10 degrees cooler the
yesterday. CAPES are progged to increase however as
east/southeasterly winds advect high dewpoints into the northeast
plains and urban corridor. NAM12 forecast soundings generate
anywhere from 1500-2500 j/kg of CAPE over east central and northeast
Colorado...with the boundary layer theta-e axis stretching from
Limon to Fort Collins. Precipitable water values by this afternoon
will be 1.2-1.3 inches with storm motions northwest around 15kts.
Southeasterly winds will also increase the likelihood of a Denver
Cyclone which will act as a focus for tstms south and east of
Denver. Enough shear today for a severe thunderstorm or two which is
why SPC has placed most of eastern CO in the marginal category this
aftn and evening. Main threat will be thunderstorm strong wind gusts
and a greater potential for heavy rainfall. Enough shear for hail up
to one inch as well. The 00Z experimental HRRR...the most complete
run at this time...appears to bring two areas of showers and tstms
across the cwa. The first one which is responsible for the tstms in
mountains west of Summit County early this progged to
start pushing into the Urban Corridor around 15z and into the
northeast plains by 20z...then another round develops from west to
east after 22Z. Not sure if this will pan out exactly as it has been
advertised...but will go with chc pops over the mountains and Palmer
Divide...with lesser pops towards southeastern WY/western Nebraska
where the boundary layer will be a bit more stable by this aftn.
Best window for tstms this evening will be in the 00z-04z
windows...with decreasing pops thereafter.

.LONG TERM...(Monday through Saturday)
Issued at 230 AM MDT Sun Jul 24 2016

The upper level high over the srn Rockies will rebuild further north
into srn CO on Mon with wnw flow aloft over nrn CO. There are some
differences between the ECMWF and other models with regard
subtropical moisture. The ECMWF shows an increase in mid lvl
moisture across the area by aftn while the other models do not.
For now will continue with 20% to 30% pops over the higher terrain
with the best focus along and south of I-70. Over nern CO the low
lvl flow will become more sse with the hi res models showing a
Denver cyclone developing. Overall low lvl moisture should
continue over the plains with decent cape. Should see wdly sct
aftn tstms along the front range with some of the storms spreading
across the plains fm late aftn thru the evening hours. As for
highs readings will be in the lower to mid 90s across nern CO.

On Tue the upper level high will still be over CO however the ECMWF
and GFS still show a wk mid lvl trough moving across late in the
aftn into Tue night. If this feature is real then there would be a
better chc of storms across the area. As for highs will keep
readings once again in the lower to mid 90s across the plains.

By Wed the upper level high retrogrades back into the Great Basin
with the flow aloft becoming more wnw.  This would keep the flow of
subtropical moisture to the south of the area and decrease the chc
of storms.  Meanwhile there are some differences between the ECMWF
and GFS with respect to the sfc ptrn across nern CO.  The ECMWF has
a cool fnt moving across the plains while the GFS does not.  As a
result this leads to significant differences in high temps across
nern CO as the GFS has readings roughly 5 to 8 degrees warmer than
the ECMWF.  In addition the ECMWF has some convection developing
over the plains as low lvl moisture increases behind the fnt while
the GFS has nothing.  At this point will mention some low pops and
keep highs in the lower to mid 90s over nern CO.

For Thu and Fri the upper level high will remain over the Great
Basin with wnw flow aloft.  Thus the flow of subtropical moisture
will stay south of the area with limited tstm development in the
mtns and higher valleys.  Meanwhile over nern CO a fnt will move
across the area on Thu with significant temp differences between the
ECMWF and GFS once again.  The ECMWF has highs only in the upper 70s
to lower 80s across the plains while the GFS has readings in the 85
to 90 degree range.  As for tstms both models show a disturbance
embedded in the wnw flow which combined with decent instability
across the plains may lead to a better chc of storms.

On Fri the ECMWF and GFS are in better agreement with highs over
nern CO as both models have readings fm 85 to 90 degrees.  Meanwhile
with decent instability remaining over the plains will likely see
another round of sct storms.  In the mtns and higher valleys tstm
coverage will likely remain rather isold.  For Sat the GFS begins to
shift the upper level high eastward into CO while the ECMWF keeps it
over the Great Basin.  Meanwhile neither model shows much influx of
subtropical moisture so tstm coverage in the mtns will remain rather
isold.  Over nern CO there still will be favorable instability in
place so will mention widely sct storms.  Highs on Sat will be in
the upper 80s to lower 90s across the plains.


.AVIATION...(For the 18Z TAFS through 18Z Monday afternoon)
Issued at 1207 PM MDT Sun Jul 24 2016

VFR through tonight. Expecting isolated thunderstorms to move off
the mountains this afternoon. Main impact will be one or two
periods of variable winds gusting as high as 30 knots in the late
afternoon and evening. There is a chance of increased thunderstorm
activity in the evening, but most of this is expected to be east
of Denver so just a low probability of any greater impact to the
Denver terminals.


.BOU Watches/Warnings/Advisories...


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