EAS 486 Lecture Content for Day 20: Warm Season Climatology

1. Hagemeyer (June 1991, WAF)
   1. Assessment of the penetration of Gulf moisture into the Continental US during the warm part of the season
      1. Consequences of penetration into various portions of the US
   2. Unique analyses
      1. Evades the problem of analysis over complex topography
   3. Mean monthly analyses computed from 15-year averages
   4. Surface Analyses (Figure 2)
      1. Variables used
         1. Used potential temperature: corrects for elevation
         2. Don't have to worry about breakdown of isentropic processes
            1. Mean monthly values won't be saturated
            2. Mean monthly values won't be unstable for upright (buoyant) convection
         3. Used mixing ratio
            1. Dew point not conserved with elevation (same w will have lower Td at higher elevation)
      2. See average set-up for "Tornado Alley" Spring Outbreaks
         1. Increased moisture availability
            • 12 g/kg moves from central Gulf and south Florida (March) to central Texas through Georgia (May)
         2. Large w gradient in Texas April and May (set-up for dry line)
         3. Heating of Mexican Plateau
            • Theta = 20-24°C over high terrain of northern Mexico
            • Not too high (which would provide average cap)
      3. Increasing moisture available for East Coast thunderstorms
      4. KIAD w increases from < 4 g/kg (March) to 13 g/kg (July and August)
      5. Moisture source for Continental west of Divide changes
         1. Nearest moisture supply is Pacific Coast (March-May)
         2. Surge of moisture up Gulf of California in summer
            1. Empalme (east coast of Gulf of California) has w = 6-8 g/kg in spring
            2. Empalme has w of 12 g/kg in June, 18 g/kg in July and August
      6. Surge of moisture in southern Intermountain region accompanied by movement of hottest air from northern Mexico into New Mexico during the summer
      7. Combined 5 and 6 => Mexican (and Southwestern US) monsoon
      • NWS KPHX and KTUS monsoon tutorial
      • Pre-Monsoon Day Surface Upper-Air Sounding
      • Monsoon Day July 29, 2003 Surface Upper-Air Sounding
      8. Moisture minimum moves from Northern Plains (< 2 g/kg in March) to central Intermountain Region during summer (over Nevada June-August)
      9. Heat source moves northward and intensifies
         • Theta = 20-24°C over high terrain of northern Mexico moves into New Mexico and Arizona and increasing to Theta = 32°C during the summer
         • Cap in central and southern Plains for most of the summer
   5. 750 mb analyses (Figure 3)
      1. Analysis shows mean wind, theta, and w
      • 750 mb selected to be above height of most elevated radiosonde station
      2. Monsoon Evidence
      1. Wind flow reverses
         1. Northwesterly in intermountain region embedded in mid-latitude westerlies during spring (weakens in March)
         2. Summer dominated by southwest winds across Baja California, Mexico, and Arizona and New Mexico
         3. Subtropical high sets up along 28°N with ridge line from Arizona to Georgia/Florida
      2. Mexican Plateau goes from low w (2-3 g/kg) in spring to high value (8 g/kg) in summer
      3. Heat redistributed
         1. Temperature mostly varies by latitude in spring
         2. Hottest in Southern Rockies centered on Four Corners area during summer
         3. Thermal low develops in northwestern Mexico during the summer
      3. Mesoscale Convective Complex mechanism
   1. Region of mean implied warm-air advection moves from Kansas (May) to Nebraska-Iowa-eastern Kansas-Missouri (June) to Northern Plains (Montana to western Wisconsin) in July-August
      • Primary synoptic-scale forcing mechanism for MCC
   2. Ridge of high moisture moves from southwest Kansas (June) to Black Hills-West Kansas (w = 5-6 g/kg) in August
   4. Influence of tropics
      1. As high moves northward from Cuba (May) to northern Florida (August), stream of easterlies appear in eastern Gulf
      2. Extension of trade winds to Louisiana Coast
      • Weak wind belt to east, so tropical flow can come even further westward
      3. Possibility of tropical cyclone activity moving into Gulf from Carribean or tropical Atlantic
      4. Possibility of interaction between mid-latitude westerlies and trade wind moisture along central Gulf Coast in August and September (rainfall maximum along Texas South Coast in August-September)
      5. Trade wind inversion (really trade wind stable layer)
         1. Second w = 6 g/kg appears in July and August in Gulf of Mexico, so w lower in southern Gulf, Cuba
         2. Suppresses convection
   5. Sources of dry air aloft
      1. Northern Pacific
      2. Trade winds
      3. Great Lakes
   6. Role of Trade Wind Inversion in Severe Weather Case
1. Trade wind stable layer carried northward.
2. Air underneath increases instability.
3. When cap weakens, particular severe weather event in the Ohio Valley
4. Not sure how frequently this occurs.

Last updated: April 24, 2009

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