Severe Thunderstorm Warning 249, NWS Denver/Boulder, CO

Area Forecast Discussion
National Weather Service Denver/Boulder CO
823 PM MDT Tue Jul 4 2023

.UPDATE...
Issued at 812 PM MDT Tue Jul 4 2023

Convergence of moisture/north winds behind the cold front and the
QG lift moving in from the west fired off Denver storms as
expected this evening. The current convection will drift east and
there should be a break in the storms for Denver and the Front
Range cities. There is upstream activity from North Park to Vail
but it's much weaker and may wind up being scattered rain showers
when it arrives. This should be a 1 to 2 hour gap well timed for
many evening activities. The threat of severe weather should be
pretty low behind the strong storms that are along I-25 now.

Only minor changes needed to the forecast, mainly a little bump up
to the PoPs later tonight into Wednesday. This will mainly be for
elevated light showers, though there could still be some thunder.

&&

.SHORT TERM...(This evening through Wednesday)
Issued at 104 PM MDT Tue Jul 4 2023

...Slight risk of severe thunderstorms through this evening much
of northeast Colorado...
...Flash flooding potential exists through this evening...
...Much cooler Wednesday, with a slight risk of severe Palmer
Divide...

Overall, the timing of the forecast appears to be on track. The
upper level trough axis over the northern Rockies will move across
Wyoming this evening. As it does, a 60 kt upper level jet max over
northern UT will move across the WY/CO border by 00z this evening
which should help enhance the developing storms this evening. In
addition there will be a broad area of weak QG ascent in place
across northern CO this evening. The next cold front is moving
has moved passed Douglas and Casper WY this afternoon, and those
stations have dropped in the 50s after fropa. The front should
reach the Denver area in the 5-6 pm time frame. SPC mesoanalysis
still indicates ML CAPE/Sfc Based CAPES of 1000/2000 j/kg across
the urban corridor and northeast plains respectively. The storms
are starting to intensify and will continue to do so into this
evening. The simulated reflectivity of the NAMNST model appears
best reflect the current situation this afternoon. It keeps the
keeps the showers and storms active through the evening, with some
gradual decrease in intensity from west to east after midnight.
The main threat from the storms will be damaging wind and hail,
with a lesser threat of tornadoes. Localized flash flooding can't
be ruled out due to training cells. The greatest area of concern
in the burn areas will be for Cameron Peak, but flash flooding
will not necessarily be confined to that area. precipitable water
values will increase significantly overnight, generally 1.0 near
the foothills to 1.3 inch over the far northeast plains.

On Wednesday, it will be socked in the stratus at least through
the morning. High temperatures will struggle to reach the mid
60s in Denver. The record low max of 62 in 1960 should stay in
tact but it will be close. Last year on July 5th, we topped out
at 96F. There will be a threat of severe thunderstorms, but it
should be confined to the Palmer Divide. Further north, it will be
much more stable with more showers vs thunderstorms. Overall the
best chance of storms will be over the elevated terrain.

.LONG TERM...(Wednesday night through Tuesday)
Issued at 104 PM MDT Tue Jul 4 2023

Cross sections show low clouds hang on through the day and into
Wednesday evening and night across the Front Range and eastern
plains. It should remain too stable for thunderstorms for most
locations. The Palmer Divide and into Lincoln county could see
some thunderstorms where the lower clouds may erode and
temperatures warm more. Storms here could become severe with large
hail and heavy rain accompanying the storms.

Strong west to northwest flow aloft will prevail Thursday through
Saturday around an upper level ridge that's been pushed south and
will be along the U.S./Mexico border. Low clouds are expected to
dissipate Thursday morning. Temperatures will warm up into the
upper 70s and lower 80s with the increased sun. This will help to
destabilize the airmass with surface based CAPE of 1000-2000 J/kg.
Higher surface pressure over the Northern and Central Plains will
keep an easterly low level flow over the Front Range and eastern
plains. These easterly winds will continue to advect moisture into
the area. Westerly winds above the low level easterlies will
result in good shear to help severe storms form. These conditons
will continue into Friday and Saturday. Temperatures may be
slightly warmer by Saturday, but otherwise don't expect much
change in the weather for Friday and Saturday. The easterly winds
will not push west of the Continental Divide, so moisture will be
lacking for thunderstorms here.

For Sunday through Tuesday, the upper level ridge that's been
over the Desert Southwest will intensify. Flow aloft weakens as
the ridging begins to influence the weather more over Colorado.
This will bring warmer and drier conditions. Expect highs to push
into the mid to upper 80s for Sunday across northeast Colorado.
Warming trend continues into next week with lower 90s possible for
most locations over northeast Colorado by Tuesday. Chances for
thunderstorms will decrease. Expect isolated thunderstorms each
day Light/moderate rainfall and gusty outflow winds are expected
to accompany the thunderstorms.

&&

.AVIATION...(For the 00Z TAFS through 00Z Wednesday evening)
Issued at 812 PM MDT Tue Jul 4 2023

The main area of thunderstorms will move out of the Denver area by
0330z. Scattered showers are expected overnight and Wednesday,
and an isolated thunderstorm is still possible mainly this evening
and again Wednesday afternoon. MVFR ceilings will develop by 06z
and persist through Wednesday. Localized IFR ceilings are
possible until around 18z.

&&

.HYDROLOGY...
Issued at 104 PM MDT Tue Jul 4 2023

The moderate risk of flash flooding will still be in place
primarily for the Cameron Peak through this evening, and that
could be the case for Wednesday as well. In addition to
southeasterly upslope winds there will also be better precipitable
water values over the area. For the other burn scars the threat
will be more limited. Outside of the burn scars, the threat threat
of flash flooding will exist but should be more localized.

Easterly low level flow will keep a moist airmass in place through
Saturday. Precipitable water values will be around an inch or
slightly greater across Front Range and eastern plains. Expect
scattered to numerous thunderstorms each afternoon through Saturday.
These storms will be capable of heavy rainfall which could lead to
flooding, mainly for the flood prone areas. Airmass begins to dry
some Sunday and into next week. This will lead to fewer and weaker
storms.

&&

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

UPDATE...Gimmestad
SHORT TERM...Cooper
LONG TERM...Meier
AVIATION...Gimmestad
HYDROLOGY...Cooper/Meier

Hazardous Weather Outlook
National Weather Service Denver/Boulder CO
947 AM MDT Tue Jul 4 2023

COZ030>051-051600-
Jackson County Below 9000 Feet-
West Jackson and West Grand Counties Above 9000 Feet-
Grand and Summit Counties Below 9000 Feet-
South and East Jackson/Larimer/North and Northeast Grand/
Northwest Boulder Counties Above 9000 Feet-
South and Southeast Grand/West Central and Southwest Boulder/
Gilpin/Clear Creek/Summit/North and West Park Counties Above
9000 Feet-Larimer and Boulder Counties Between 6000 and 9000 Feet-
Jefferson and West Douglas Counties Above 6000 Feet/Gilpin/Clear
Creek/Northeast Park Counties Below 9000 Feet-
Central and Southeast Park County-
Larimer County Below 6000 Feet/Northwest Weld County-
Boulder And Jefferson Counties Below 6000 Feet/West Broomfield
County-
North Douglas County Below 6000 Feet/Denver/West Adams and
Arapahoe Counties/East Broomfield County-
Elbert/Central and East Douglas Counties Above 6000 Feet-
Northeast Weld County-Central and South Weld County-Morgan County-
Central and East Adams and Arapahoe Counties-
North and Northeast Elbert County Below 6000 Feet/North Lincoln
County-
Southeast Elbert County Below 6000 Feet/South Lincoln County-
Logan County-Washington County-Sedgwick County-Phillips County-
947 AM MDT Tue Jul 4 2023

...Severe storms possible this afternoon and evening...

This hazardous weather outlook is for northeast and north central
Colorado.

.DAY ONE...Today and Tonight

Numerous showers and thunderstorms are expected this afternoon and
tonight over the foothills and plains. Some of the storms could
be severe with large hail, damaging winds and heavy rain. Heavy
rainfall could also cause flash flooding problems, most notably
over the burn areas. There could be severe rounds of storms today,
with the second round coinciding with fire work displays and
picnics this evening. Holiday enthusiasts are urged to have
adequate ways to take cover during their outside festivities.

.DAYS TWO THROUGH SEVEN...Wednesday through Monday

Showers and thunderstorms are likely over the mountains on
Wednesday with mainly scattered showers over the plains. There
will be a threat of flash flooding over the alpine burnt areas.

Scattered late day showers and thunderstorms are forecast for
Thursday through Saturday with hail and heavy rainfall possible.
Chances for thunderstorms decrease Sunday and into early next
week.

.SPOTTER INFORMATION STATEMENT...

Spotter activation will be needed after 1 pm this afternoon.
Reports of hail, heavy rain and flooding will be appreciated.

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Special Weather Statement
National Weather Service Denver CO
750 PM MDT Tue Jul 4 2023

COZ039-040-043-050215-
Central and South Weld County CO-
Boulder And Jefferson Counties Below 6000 Feet/West Broomfield County
CO-
North Douglas County Below 6000 Feet/Denver/West Adams and Arapahoe
Counties/East Broomfield County CO-
750 PM MDT Tue Jul 4 2023

...Strong thunderstorms will impact portions of southwestern Weld,
western Arapahoe, Broomfield, Denver, northeastern Jefferson, western
Adams and eastern Boulder Counties through 815 PM MDT...

At 750 PM MDT, Doppler radar was tracking strong thunderstorms along
a line extending from near Mead to Denver, or along a line extending
from 20 miles southwest of Greeley to 55 miles north of Colorado
Springs. Movement was east at 30 mph.

HAZARD...Wind gusts up to 50 mph and penny size hail.

SOURCE...Radar indicated.

IMPACT...Gusty winds could knock down tree limbs and blow around
         unsecured objects. Minor damage to outdoor objects is
         possible.

Locations impacted include...
Denver, western Aurora, Lakewood, Thornton, Arvada, Westminster,
northern Centennial, Longmont, Broomfield, Commerce City,
northeastern Littleton, Northglenn, Brighton, Englewood, Wheat Ridge,
Lafayette, eastern Golden, Louisville, Erie and Greenwood Village.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

If outdoors, consider seeking shelter inside a building.

Torrential rainfall is also occurring with these storms and may lead
to localized flooding. Do not drive your vehicle through flooded
roadways.

A Severe Thunderstorm Watch remains in effect until 1100 PM MDT for
central and northeastern Colorado.

&&

LAT...LON 3962 10471 3961 10473 3959 10474 3960 10515
      3963 10516 3965 10519 3967 10519 3968 10515
      3969 10514 3972 10515 3973 10520 4024 10512
      4024 10463
TIME...MOT...LOC 0150Z 279DEG 27KT 4020 10501 3963 10509

MAX HAIL SIZE...0.75 IN
MAX WIND GUST...50 MPH

RPK