Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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FXUS65 KBOU 222117
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
317 PM MDT Sun Jul 22 2018

.SHORT TERM...(This evening through Monday)
Issued at 317 PM MDT Sun Jul 22 2018

Convection will continue to spread eastward through the afternoon
and evening. Like yesterday, expect outflow boundaries to trigger
much of the convection. Gusty winds to 40 mph and brief heavy
rain will be the main threats along the Front Range. Over the
eastern plains, roughly east of a line from Sterling to Akron ML
CAPE reaches 1000-2000 J/kg. This should be enough to fuel a
couple of severe thunderstorms.

North winds behind a cold front will turn northeast to easterly
tonight and Monday, increasing low level moisture. At the upper
levels, a weak wave embedded in the westerly flow aloft combined
with the upslope flow will provide lift to keep scattered/isolated
showers and thunderstorms going through the night.

The NAM, RAP, ECMWF, and GEM all show precipitable water values
late tonight and Monday ranging from 1.25 to 1.5 inches across the
Front Range and northeast Colorado. The GFS is a little drier and
quicker to bring the drier air off to the north into the area.
Expect the easterly upslope flow to help keep moisture over the
area through the day, keeping chances for showers and
thunderstorms high. Because of the high precipitable water values,
heavy rain and flooding will be possible. The ECMWF shows 1 to 3
inches of rain along the Front Range for late tonight and Monday,
but the GFS generally has less than a quarter inch. We may need a
Flash Flood Watch for Monday. However, due to large differences
between the models and a good amount uncertainty, will hold off at
this time.

With all the moisture around, mostly cloudy skies are expected
for Monday and will keep highs in the 70s for the Front Range and
most of the plains. If remains overcast all day, some locations
may stay in the 60s. If cooler temperatures prevail, this will
help to stabilize the airmass and lower the chances for storms.

.LONG TERM...(Monday night through Sunday)
Issued at 317 PM MDT Sun Jul 22 2018

A cooler and wetter week looks to be in store across northern
Colorado. Medium range models continue to show the upper level
ridge over the southwest U.S. moving westward through the week
with abundant mid-level moisture moving over the central Rockies
and Colorado. Along with that, surface high pressure will be in
place over the northern Great Plains, causing periods of upslope
low level flow across northeast Colorado. Moisture fields indicate
that precipitable water values will spend most of the week in the
1.0 to 1.25 inch range. Monday night will be moist from the
foothills across the plains as a follow-up the cool and moist
afternoon. Tuesday will be slightly drier and a little warmer,
depending on how much rain falls Monday afternoon to moisten the
soil levels. High temperatures should reach the upper 80s, but
with mid-level moisture over the state, warming of the day may
produce enough instability for another round of afternoon
convection.

From Wednesday through the end of the week, the models show that
the upper level jet on the north side of the upper ridge will
begin sagging over northern Colorado. This increase in westerly
flow at high levels will help organize the afternoon storms that
are forecast each afternoon. Sub-tropical moisture is expected to
remain over the region and feed afternoon and evening
thunderstorms that roll off the mountains and across the plains.
The ECMWF and GFS have decent amounts of precipitation over the
forecast area each afternoon from Wednesday through Sunday. The
showers and cloud cover will also help to keep temperaturs cooler
than normal for the last week of July. Hopefully we get enough
rain to improve soil moisture conditions without going too far
overboard and causing flooding type issues.

&&

.AVIATION...(For the 18Z TAFS through 18Z Monday afternoon)
Issued at 317 PM MDT Sun Jul 22 2018

Scattered thunderstorms will move across the Denver area this
afternoon and evening, beginning 20-21Z. Visibility may fall under
5 miles under the heavier rain. Main issue will be wind shifts
due to outflow winds from convection. Gusts to 30 knots will be
likely with a slight chance for wind gusts to 40 knots.

North to northeast winds tonight combined with increasing
moisture will produce low clouds tonight and Monday. In addition,
a period of rain will be possible. Ceilings are expected to fall
to 2000 to 4000 feet. The low clouds could persist for most of
Monday.

&&

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

$$

SHORT TERM...Meier
LONG TERM...Dankers
AVIATION...Meier



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