Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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337
FXUS65 KBOU 272206
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
406 PM MDT Thu Jul 27 2017

.SHORT TERM...(This evening through Friday)
Issued at 400 PM MDT Thu Jul 27 2017

An abnormally moist atmosphere trapped beneath a large upper level
high pressure will remain over Colorado/eastern Utah through at
least Friday. Based on radar and satellite imagery, convection has
been quite limited this afternoon. Only in the past hour have t-
storms begun to form over the northern Front Range and over and
around South Park. Believe the tranquility elsewhere will break in
the next hour or two. A moist southeast low-level flow on the
plains, steep 700-500mb lapse rates and the passage of a weak
disturbance in light northwest flow aloft are still expected to
generate t-storms scattered across eastern sections of the CWA
through this evening. Storms in the South Park area, of which one
has produced hail in the past several minutes, are predicted to
track east-northeast over southern portions of the I-25 corridor by
00z. This batch of storms is then projected to merge this a broken
line of t-storms swinging out of the northern foothills in the next
hour or so and then southeast across ern Larimer, Weld and Morgan
counties 00z-04z this evening. OOz/Sat RAP and NAM sfc CAPES in this
area anywhere from 1500-2500j/kg. Boundary layer shear also looks
favorable for storm formation. Where these two batches of storms
come together, best estimate in the eastern Weld, Morgan and Logan
county area, could see some storms turn severe. SPC has part of this
area in a slight risk for severe. Large hail and damaging winds main
svr threats. Slow storm motions and mean layer PWs in the 1.0 to 1.2
range could also result in heavy rainfall from a few of these cells,
with locally up to 1.5 inches in under 45 minutes. Most if not all
of this storm activity is forecast to exit to the CWA around
midnight. Could see patchy fog form again in wet low-lying areas
towards dawn. But, not as widespread as this morning.

On Friday...these same warm and humid conditions beneath the under
ridge will be in place for another round of t-storms by the
afternoon. PW values are forecast to gradually increase through the
day, setting the stage for locally heavy rainfall once again from a
slow moving storms. The formation of a Denver cycle tomorrow
afternoon could also create conditions suitable for hail in the
Denver metro area. Will need to monitor this closely tomorrow.
Otherwise, temperatures during the next 24 hours will remain below
average.

.LONG TERM...(Friday night through Thursday)
Issued at 400 PM MDT Thu Jul 27 2017

The upper ridge axis will stay right over Colorado Friday night
into Saturday. The upper ridge is to migrate westward and be over
the Great Basin by Sunday morning. There is little in the way of
any synoptic scale energy through Sunday night. The low level
winds look to stick to normal diurnal trends. Precipitable water
values remain in the 0.75 to 1.50 inch range all five periods. Dew
point readings range from the upper 40s F west to mid 60s F east
through Sunday night. There are pretty decent CAPE values over the
eastern two-thirds of the CWA Friday evening into Saturday
afternoon. The best CAPE for late day Sunday is over the far
eastern CWA. The QPf fields show pretty decent coverage of
measurable rainfall Friday evening, then again Saturday afternoon
and much of the night. Again, Sunday afternoon and evening there
is decent QPF. Will leave in or go with "likely"s in the
mountains, with "chance"s over the plains. Will also need pops in
the overnight and morning periods. For temperatures, Saturday`s
highs are 1-3 C cooler than Friday`s. Sunday`s readings are
similar to Satruday`s but a bit cooler of the northeast corner.
For the later days, Monday through Thursday, the upper ridge axis
stays west of Colorado Monday. By Tuesday and continuing through
Thursday, the upper ridge is centered over the Great Basin into
southern Idaho. There is moisture around through Tuesday night,
then it dries out Wednesday quite a bit. Temperatures stay below
seasonal normals.

&&

.AVIATION...(For the 18Z TAFS through 18Z Friday afternoon)
Issued at 400 PM MDT Thu Jul 27 2017

VFR conditions may be interrupted by scattered thunderstorms
tracking east-northeast across the Denver metro area between
00z-02z this evening. Then a second bath of storms up in Weld
County is forecast to clip the northeast half of the metro area
sometime between 02z-04z this evening. Both waves of storms could
produce brief heavy rainfall, wind gusts to 40 kts and frequent
lightning. Can rule out small hail with the stronger storms.
IFR vsbys and MVFR ceilings possible with the heavy rainfall.
The storm threat should end in the metro area before midnight,
with VFR conditions and light southerly winds for the rest of the
night. On Friday, VFR conditions with light southerly winds in
the morning. then east-northeast winds of 8-15 kts in the
afternoon with scattered thunderstorms forming in the area after
20z. Locally heavy rainfall, strong gusty outflow winds and even
small hail will be possible.

&&

.BOU WATCHES/WARNINGS/ADVISORIES...
None.
&&

$$

SHORT TERM...Baker
LONG TERM...RJK
AVIATION...Baker



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