convective forecasting issues in the southwestern united states monsoon regime challengesmonsoon...

Convective Forecasting Issues in the Southwestern United States

• Monsoon Regime ChallengesMonsoon Regime Challenges

• Monsoon Climatology for Las VegasMonsoon Climatology for Las Vegas

• The Las Vegas Convergence ZoneThe Las Vegas Convergence Zone

• Classic Monsoon Regime PatternsClassic Monsoon Regime Patterns– Flash Flood Signatures Flash Flood Signatures (8 JUL 99)(8 JUL 99)– MCS Signatures MCS Signatures (7-8 AUG 97)(7-8 AUG 97)

• Transition Case Patterns Transition Case Patterns – An organized SVR/FFW Event (10 AUG 97)An organized SVR/FFW Event (10 AUG 97)

Monsoon Regime Challenges

• continual fluctuation between subtropical easterlies and polar westerlies

• poor sampling of short waves in easterlies

• relatively poor density of surface data

• typically low-shear environment (therefore, the primary ingredient = thermodynamics)

• storm-relative inflow of buoyant air may be as important as cold pool-shear balance… but usually difficult to assess accurately

The Monsoon Season in Las Vegas

ThunderstormClimatologysuggests the

“normal season”runs from aboutJuly 7 - Sept 11

with peak activityJuly 15 - August 15

Diurnal Summer Precipitation Frequency

Most local thunderstorms occur in the mid to late afternoon with a secondary peak around midnight

Precipitation Amount at LAS

Less than 1 percent of the total number of LAS summertime precipitation events exceed 0.25”

Exceptional Storm Totals

• 2.59” (8/21/57)• 1.75” (8/10/42)• 1.56” (8/12/79)• 1.36” (7/28/84)• 1.34” (8/17/77)• 1.32” (7/24/56)• 1.29” (7/24/55)• 1.25” (7/26/76)

• 3.19” (7/8/99) Blue Diamond Ridge

• 3.13” (8/10/97) Boulder City

• 2.24” (9/11/98) Meadow Valley Wash

• 2.05” (7/19/98) Flamingo Wash

• 1.89” (9/11/98) California Wash

At McCarran: Within Clark County:

Surface Dewpoint Trends

Dewpoints on thunderstorm days average ~55 F and plateau or rise from late afternoon into evening; non-thunderstorm days

normally remain < 45 F and decrease throughout the afternoon.


In contrast to the steady drying observed on non-thunderstorm days, dewpoints tend to rise substantially for 1-2 hours prior to

the onset of a mid-afternoon thunderstorm at McCarran.


Similarly, midnight thunderstorms tend to be preceded by a noticeable increase in surface dewpoint during a period

when the normal diurnal trend would suggest drying.

Southern Nevada Thunderstorm Days(average morning sounding parameters)

• deep, well-mixed elevated boundary layer• 700-500mb lapse rate > 7 C km-1

• surface-700mb theta-w > 17 C (mean mxr > 8 g kg-1)

• average 12Z CAPE only about 250-300 J kg-1

• modest deep-layer (0-6km) shear

• propagation into valleys dependent on:• mean wind in the cloud-bearing layer

• ambient vertical wind shear

• bouyancy of the surface inflow layer

Forecasting Problems

• DRA often not representative of LV valley• model soundings typically not very valuable• convective structure/evolution sometimes

modulated by local circulations • what buoyancy/shear values signal potential

for organized convection vs. isolated storms?• how can forecasters assess the influence of

storm-relative inflow and internal feedback processes which alter the ambient conditions?

The Las Vegas Convergence Zone

Las Vegas Valley Orography

Reflectivity Image of LVCZ Event

LV mesonet surface winds superimposed on KESX WSR-88D comp reflectivity, valid 2039 UTC, 30 July 1997.

Purpose of This Study

Reproduce classic LVCZ circulation in numerical simulations

Isolate important processes by systematic variations in wind and thermal profiles

Summarize findings for use in operational convective forecasts

RAMS Simulations

Initial real time simulations failed• grid-scale convection overwhelmed signal• horizontal resolution perhaps too coarse

Subsequent reruns successful• inner nest contracted to 4-km• turned off convection

Sole exception = dry simulation with no diurnal radiation cycle

RAMS Configuration

Non-hyd, 2-way interactive nest (12-km/4-km) 25 vertical layers, 1-km NCAR terrain 30-sec time steps, full microphysics Mahrer-Pielke radiation, no convec. param. First runs initialized with real data; lateral

boundaries nudged toward Eta-29 or RUC Second runs horizontally homogeneous;

modified proximity sounding from DRA

Sounding Profile for HH RunsMean 1-4 km wind ~ 230/06 ms-1

Surface Winds and Convergence: 1800 UTC

Tilt View of LVCZ Circulation

Surface wind vectorsand shaded terrain in bottom half of image.(LVCZ highlighted)

Potential temperaturecontours and shadedU-component of windin upper half of image.(vertical solenoid onlee-side of Mt Chuckhighlighted by arrows)

Surface Winds and Convergence: 2100 UTC

Surface Wind & Relative Vorticity: 1800 UTC

8/23 storm as it intersected CZ: VIL=30 kg m-2

8/23 storm 12 minutes later (VIL = 67 kg m-2)

Reflectivity Cross-Section: 2156 UTC (corresponds to time of 67 VIL)

Reflectivity Cross-Section: 2208 UTC (corresponds to 90kt downburst and 1-inch hail 3” deep)

Photo of downburst ~ 2210 UTC

2100 UTC Surface Wind & Convergence with Mean 1-4 km Wind ~ 230/12 ms-1

2100 UTC Surface Wind & Convergence with Mean 1-4 km Wind = 180/07 ms-1

2100 UTC Surface Wind & Convergence with Mean 1-4 km Wind = 310/07 ms-1

Composite Reflectivity - 26 May 1999

Key Factors for Classic LVCZ

• Mean wind in the 1-4 km layer AGL:– direction between 200 and 280 degrees– speed less than 10 ms-1 (optimum ~5-7 ms-1)

• Deep, well-mixed unstable boundary layer elevated above a shallow, surface-based inversion in the early morning hours

• Surface dewpoints > 60 F (PW > 1.00”)

• Afternoon CAPE > 1500 J kg-1

Conceptual Diagram of the LVCZ life cycle.

Conclusions• The classic LVCZ is the result of an interaction

between the ambient wind and a thermally forced mountain-valley circulation under a specified range of prerequisite conditions.

• Range of conditions is now understood better and forecaster awareness raised; under such conditions, deep moist convection is likely and potential for explosive growth is substantially increased.

• Further study is required to develop more definitive parameters w.r.t. local effects of stability and minimum moisture needed to support deep convection in the Las Vegas valley.

Classic Flash Flood SignaturesIllustrative Case: July 8, 1999

Organized Severe Storms

Illustrative Cases: August 7-8, 1997

Eta 00h: 500mb flow & 700-400mb omega

Eta 06h: 500mb flow & 700-400mb omega

Eta 12h fcst: 500mb flow & 700-400mb omega

Eta 00h: CAPE, CIN, & 850mb Theta-e

8/12Z GOES WV & RAMS 500mb Wind/Temp

Eta 00h 500mb flow & 700-400mb omega

Eta 00h CAPE/CIN & 850mb Theta-e

8/21Z GOES VIS & SFC OBS

9/00Z GOES-9 IR IMAGERY

KIWA 8/2303Z Composite Reflectivity

Organized Severe Storms:A Transition Case

~ 10 August 97 ~

GOES-9 Visible Image: 14Z - 10 Aug 97

GOES-9 IR Image: 19Z - 10 Aug 97

GOES-9 IR Image: 21Z - 10 Aug 97

12Z Eta 00h 310K theta surface & mixing ratio

Eta 12h forecast 850mb theta-e & wind

GOES-9 Sounder CAPE: 20Z – 10 Aug 97

GOES-9 Sounder LI: 20Z – 10 Aug 97

Base velocity - 10/1922Z

Schematic of System Propagation

Composite Reflectivity: 2020Z – 10 Aug 97

Radar echoes & surface streamlines – 2030Z

VIL & Storm Track – 2020Z

VAD Wind Profile – 1922-2020Z

VAD Wind Profile 0024-0122Z

Storm Total Precipitation – 10 August 1997

convective forecasting issues in the southwestern united states monsoon regime challengesmonsoon...

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