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Review of DAG Winter Weather Forecasting and NW Flow Snow Web Pages

Review of DAG Winter Weather Forecasting and NW Flow Snow Web Pages. Brian LaSorsa and Woody! LWX Media Workshop 12/09/11. SYNPOTIC OVERVIEW.

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Review of DAG Winter Weather Forecasting and NW Flow Snow Web Pages

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  1. Review of DAG Winter Weather Forecasting and NW Flow Snow Web Pages Brian LaSorsaand Woody! LWX Media Workshop 12/09/11

  2. SYNPOTIC OVERVIEW • Dave Gustin (DAG) came up with several theories that can be applied to winter weather forecasting at LWX. Using his theories along with your own knowledge of synoptic forecasting, we will take a look at past events to see how they can be applied. • We’ll take a brief look at upslope snow with a quick review of Joe Wegman’s research. • Wrap-up

  3. 1)  A 500 MB LOW ABOUT 200 miles northeast of Newfoundland.     (For a negatively tilted trough upstream of DCA...it is better to have the 500 mb low near 45n 50w. – 200 miles southeast of Newfoundland) 2) SPLIT FLOW WITH THE SNOW MAKING UPPER LOWINITIALLY IN THE GULF COAST REGION. 3) A BUILDING RIDGE OVER THE WESTERN ROCKIES (over Idaho). (If the ridgeis west of the west coast, the surface low will probablytrack west of DCA. Consequently if the ridge is further east over the eastern Rockies, the surface low is likely to track too far to the east off the coast.) 4) A BLOCKING HIGH JUST WEST OF GREENLAND INTHE WINTER AND OVER HUDSON BAY IN MARCH. 5) THE SNOW MAKING UPPER LOW ORIGINALLY ENTERINGTHE UNITED STATES FROM MEXICO OR THE PACIFICOCEAN. IMPORTANT SYNOPTIC PATTERNS FORWINTER STORM FORECASTING FAVORABLE UPPER AIR PATTERNS (TELECONNECTIONS) FOR BIG SNOWS AT DCA.

  4. Upper Level Low south and east of Newfoundland Blocking High just west of Greenland Negatively tilted trough axis upstream Upper level ridge axis over western Rockies

  5. 3) A BUILDING RIDGE OVER THE WESTERN ROCKIES (over Idaho). Upper level ridge axis over eastern Rockies

  6. FAVORABLE SURFACE PATTERNS FOR BIG SNOWS AT DCA 1) **A LOW TRACKING NORTHEASTWARD BETWEEN ORF AND HAT, OR A LOW TRACKING NORTHWARD ALONG THE MID ATLANTICCOAST.** (Backed up by brief climatological research) 2) A LOW ORIGINALLY DEVELOPING SOUTH OF VIRGINIA    (The further south the better!!  This is so true! A low that does not originate south ofVirginia rarely produces heavy snow at DCA.) 3) THE LOW RAPIDLY INTENSIFYING (But not always necessary) 4) A HIGH TO THE NORTH OR WEST. (Preferably north, as that willaid in the inflow off the Atlantic ocean.  Ultimately, the inflow off theAtlantic ocean determines the average rate of the snowfall.) 5)  THE PRESSURE OF THE HIGH (If it is north of DCA) GREATER THAN     1040 MBS.  THE HIGHER THE PRESSURE, THE MORE SNOW!! (cont next page)

  7. FAVORABLE SURFACE PATTERNS FOR BIG SNOWS AT DCA (cont) 6)  THE LOW MOVING VERY SLOWLY, OR NOT AT ALL. 7) A MODERATE SNOW (before a changeover to rain) IS LIKELY TO OCCUR AT DCA WHEN A LOW TRACKS NORTH OR NORTHEASTWEST OF  THE APPALACHIANS...AND THERE IS PLENTY OF ARCTICAIR IN THE MID ATLANTIC REGION.  (However ironically enough, ifthere is a strong high north of DCA...sometimes that high will delay theprecipitation getting into DCA...reducing snow amounts!!) 8) WITH INTENSIFYING LOWS,  THE SNOW USUALLY ENDSWHEN THE LOW PASSES BY DCA.  WITH NON-INTENSIFYING LOWS,THE SNOW USUALLY ENDS MUCH SOONER THAN THAT.

  8. 9z, 18z, 21z 1) A LOW TRACKING NORTHEASTWARD BETWEEN ORF AND HAT, OR A LOW TRACKING NORTHWARD ALONG THE MID ATLANTICCOAST.

  9. Low Pressure Developing North of VA/NC state line: 10z and 14z Analysis: 12/30 2000 non-event 10z 14z 2) A LOW ORIGINALLY DEVELOPING SOUTH OF VIRGINIA    (The further south the better!!  This is so true! A low that does not originate south ofVirginia rarely produces heavy snow at DCA.)

  10. The True Art of Forecasting We’ll take a look at map analyses from several winter storms over DC area, using our knowledge of synoptic meteorology lets take a wild guess as to whether there was warning snow criteria at KDCA/KIAD.

  11. Storm 1 • Looks at real time observations from 12z for each mandatory pressure level starting at 300mb. Precipitation at Washington D.C. fell mainly during the afternoon and evening hours that day.

  12. Positively tilted trough axis Upper level ridge axis over western Rockies

  13. Embedded shortwave in longwave pattern Upper level ridge axis over western Rockies

  14. Left exit of 700 mb. Also look at speed convergence and strong warm advection. 700 mb trough axis positively tilted. 700 mb ridge axis over western Rockies

  15. 23C temp/dewpoint depression shows extreme amounts of dry air. Northwest wind over Wallops Island, showing that cold air is still advancing. Low is likely to pass by to the south. Therefore, warm advection/isentropic lift at low levels will likely remain to the south of DCA Closed 850mb low

  16. Very cold/dry air Wind parallel to boundary, suggests that no warm advection will make it this far north

  17. Surface low to the southeast of Cape Hatteras. (a bit too far south per DAG’s rule for heavy snow over DC (note very cold air overhead)

  18. Storm 2 • Looks at real time data from each mandatory pressure level starting at 300mb. The data starts with 12z observations and ends with 00z observations at each level. • Most of the precipitation from the storm fell during the overnight hours after the 00z obs.

  19. Speed and directional divergence: Left exit jet streak, right entrance jet streak lined up Ridge still over western Rockies Ridge over western Rockies Negatively tilted trough axis. 300mb low has closed off. Strong diffluence aloft Strong Jet max rotating around trough

  20. Upper Level Ridge over western Rockies Upper level ridge shifted to central Rockies Upper level low lifting northeast. Strong Diffluence approaching LWX. Stronger winds on eastern side of trough axis, and divergence aloft Closed upper-level low over Gulf Coast States (DAG rule of thumb states that is a good set up for heavy snow

  21. Dry slot wrapped around 700mb low. Moisture fetch on northern side of low approaching cwa Strong jet max associated with moisture surge Mid-level ridge axis over western Rockies Cut off 700mb low. Wind rotating around trough axis, likely to become negatively tilted. Note warm and moist air over Atlantic South branch trough axis swinging toward negatively tilted.

  22. Northerly wind developed, suggesting low will pass to the south. Low level cold air in question Wind on cold side is parallel Strong jet max on northern side of low, likely to add lift. Note air is saturated over KIAD: GSO wind parallel to boundary

  23. Wind parallel to boundary, 925 mb low likely to track too far south to impact our area Northerly wind showing strong cold advection (note airmass to north) Low likely to track to the south, no isentropic lift (little jet dynamics associated with 925 mb low

  24. Surface warm air has caused precip to fall as rain early. (what is likely to occur based advection upstream? Double low, usually a sign that the storm is not organized enough for heavy snow. However, consider upper-level dynamics

  25. Storm 3 • Takes a look at real time data for 12z at each mandatory pressure level. You can use your knowledge of synoptic meteorology to determine whether or not there will be heavy snow as you go along. The answer is on the surface map.

  26. Presidents Day Storm 2003. 2/17/00z. High pressure over Southern Ontario over 1040mb Low center approx 1012 mb. Weak low, but still heavy snow over DCA. Likely due to strong isentropic lift and deep moisture. 5)  THE PRESSURE OF THE HIGH (If it is north of DCA) GREATER THAN1040 MBS.  THE HIGHER THE PRESSURE, THE MORE SNOW!!

  27. 6) THE SNOW MAKING UPPER LOW ORIGINALLY ENTERINGTHE UNITED STATES FROM MEXICO OR THE PACIFICOCEAN...

  28. A listing of the largest snowfalls, and ice storms, to impact the Sterling forecast area is on the office internet page: http://www.erh.noaa.gov/lwx/winter/DC-Winters.htm You can check out PSU’s re-analysis page to look at the synoptic charts for those events during or after 1979: http://www.meteo.psu.edu/~gadomski/NARR/index.html SOO’s car-19 Dec 2009 (note portion of tarp sticking out from side…nice idea!)

  29. Snow in the Highlands We have come a long way since the 1990’s with regard to forecasting snowfall at the higher elevations in the western part of the CWA. Summer intern Joe Wegman did much work in 2008 and ‘09. For a detailed tutorial (including a briefing on lake effect snow by BUF MIC Tom Niziol) go to: O:\Woody\Presentations\Winter Program\lakeeffect

  30. L H This shows the amount of area the Great Lakes encompasses, and how much moisture is available to modify the atmosphere on northwest flow.

  31. Checklist Analysis 7 key variables • Average Wind Direction & Speed in the BL (º, kts) • Height of the Inversion (ft) • %RH and Omega in the Snow Growth Layer • Does the DSGL exist below the inversion? • Average %RH upstream at 925 • Significant Fetch over the Great Lakes? • How much Instability is there?

  32. Evaluation Table

  33. SYNOPTIC PATTERNS FAVORING RAIN OR MIXED PRECIPITATION 1) A SURFACE HIGH EAST OF THE ATLANTIC COAST 2) AN 850 MB LOW TRACKING NORTH OR WEST OF DCA 3) IF AN ARCTIC AIRMASS IS OVER THE MID ATLANTIC REGION…AN 850 TEMPERATURE OF ZERO OR HIGHERAND/OR A 1000-500 MB THICKNESS OF 546 OR HIGHER.  THEPRECIPITATION TYPE THEN MOST LIKELY WOULD BE SLEETOR FREEZING RAIN.  (cont next page)

  34. SYNOPTIC PATTERNS FAVORING RAIN OR MIXED PRECIPITATION (cont) 4) A STRONG SOUTHEASTERLY SURFACE GRADIENT FAVORS RAIN. 5) 500 MB HEIGHTS GREATER THAN 564 DECAMETERS FAVOR RAIN. (Typical inLa Nina winters). 6) A PRIMARY LOW TRACKING NORTHEASTWARD TOWARDTENNESSEE AND A SECONDARY LOW DEVELOPING EAST OF THECAROLINAS. (When there is arctic air in the mid Atlantic region...thisis an excellent scenario for a large accumulation of sleet). For a great study on a major DC sleet storm, go to O:student volunteer\2008\Elizabeth2008\2-14-07\2-14-07.ppt

  35. 4) IF AN ARCTIC AIRMASS IS OVER THE MID ATLANTIC REGION…AN 850 TEMPERATURE OF ZERO OR HIGHERAND/OR A 1000-500 MB THICKNESS OF 546 OR HIGHER.  THEPRECIPITATION TYPE THEN MOST LIKELY WOULD BE SLEETOR FREEZING RAIN.

  36. 12Z-00Z 2/12-13 SURFACE ANALYSES

  37. 2007 Sleet/fzra storm: 2/14/00z sleet/fzra: 850mb tracked well west of DCA, but cold air was trapped at the low-levels (925mb to surface) Also note high pressure due north of DCA, great setup to keep low-level cold air in place.

  38. 6) A PRIMARY LOW TRACKING NORTHEASTWARD TOWARDTENNESSEE AND A SECONDARY LOW DEVELOPING EAST OF THECAROLINAS. (When there is arctic air in the mid Atlantic region...thisis an excellent scenario for a large accumulation of sleet). For a great study on a major DC sleet storm, go to O:student volunteer\2008\Elizabeth2008\2-14-07\2-14-07

  39. NOTE ON RAIN CHANGING TO SNOW IT IS DIFFICULT TO GET RAIN TO CHANGE TO SNOW IN WASHINGTON.HERE ARE SOME FACTORS WHICH MIGHT AID IN SUCH A CHANGEOVER... 1) IMMEDIATE EVAPORATIONAL COOLING* 2) LOWER DEWPOINTS TO THE NORTH OF DCA WHICH ARE EXPECTED    TO ADVECT SOUTHWARD (It is very hard to get lower dewpoints which arenorthwest or west of DCA to advect into our area during a precipitation event.) 3) A LARGE INCREASE IN VERTICAL MOTION* (JAN 26 2011!!) 4) IF IT IS AN OFF-SEASON STORM. (Big early March storms may beginas rain.)

  40. March 15th 2007 max temp at IAD was 77 degrees. Cold front came in the afternoon. Surface analysis 3/16 12Z: Note Arctic High to the north and lower dewpointsadvecting into the area on a northerly flow. Rain changed to snow at KIAD before ending. 1.2 inches of snow fell the day after max temps were in the upper 70s. 3/17 00Z 2007: Rain changed to snow at Washington DC and Points north and west. Northerly flow continues to advect lower dewpoints as coastal low pressure develops off the coast.

  41. Summary • Hopefully this presentation gives you a better idea of what synoptic features support what different types of winter weather around Washington D.C. • Use your knowledge of synoptic meteorology along with some of these features from the presentation to interpret guidance. (ex: computer models diverge in solution for heavy snow event, but convergence in synoptic positioning of highs/lows, see if it fits the mold for a heavy snow setup. • Model QPF is the subject to large errors, use knowledge of synoptic meteorology to either back up or disprove model qpf. (ex: 1/26/11 model qpf was very high, you had to find reasons why it was so high because it was conceivable that it was suffering from convective feedback). • Use observations and difax charts to see how guidance is initializing. In the near term, you may find out that all guidance is initializing poorly (Jan 2000), you can add significant value to computer model output.

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