South Florida Winter Outlook and Forecast Tips

Transcription

1 South Florida Winter Outlook and Forecast Tips Robert Molleda Warning Coordination Meteorologist National Weather Service Miami Forecast Office

2 South Dade Freeze Climatology Since 1948, 31 years have had freezing temperatures (32F or colder). Return period is slightly over 2 years. Longest stretch without freeze since 1948: 6 years (occurred twice and ). No freezes the past two winters

3 South Dade Freeze Climatology Since 1948, 46 years have had temperatures of 35F or lower (near freeze or freeze). 69% of years since Lowest recorded temperature in S. Dade ag area: 25F at Coop office (Dec 1989). 26F at Tamiami Airport on Jan 11, 2010.

4 Cold Outbreak Indicators High pressure from Canada centered over North Dakota/Minnesota and moving due south towards Gulf coast and Florida, with low pressure over NE U.S. or eastern Canada. Brown lines roughly depict wind flow around the two systems, in this case NW over Florida.

5 Cold Outbreak Indicators Snow cover deep into southern U.S (shown in blue here). Arctic air blowing over snow cover will not warm as fast as it moves south.

6 What Could Happen This Winter??

7 Temperature Outlook: Dec-Feb Slight tendency for cooler than normal temperatures

8 Long-range CFS Model Temperature Projections: Jan-Mar

9 Long-Range Forecast Tools ENSO (El Niño Southern Oscillation). In other words, El Niño/La Niña This winter, we are expected to be in the El Niño phase (most likely a weak event) Other factors play a big role in winter weather pattern across N. America, such as North Atlantic Oscillation and Pacific-North American Pattern

10 El Niño/La Niña and South Florida Freezes 31 freeze seasons since Neutral 12 of 24 (1 in 2) - La Niña 11 of 21 (1 in 1.9) - El Niño - 8 of 21 (1 in 2.6)

11 North Atlantic Oscillation Slide Courtesy: NWS Melbourne, FL Periodic Fluctuation of Pressure Patterns over the North Atlantic Ocean Scale of Weeks Primary Impact on Winter Temperature Positive Warm Negative Cold

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13 South Florida Winter Precipitation Outlook Likelihood of wetter than normal for Florida

14 NWS Cold/Freeze Products

15 NWS Freeze Products Freeze Outlook---issued 3-5 days before a freeze is forecast. Freeze Watch---issued hours before a freeze is forecast. Freeze Warning---issued hours before a freeze is expected. Hard Freeze 28F or below for 4 or more hours.

16 NWS Frost Products Frost Advisory---issued if scattered to widespread frost is expected within hours. Temperatures 33-37F with clear skies, winds below 5 mph, and sufficient low level moisture.

17 NWS Wind Chill Products Wind Chill Advisory- issued if wind chill expected to drop to below 35F for at least 4 hours (need wind of 5 mph or greater). Wind Chill Watch/Warning issued for wind chill 25F or lower for at least 4 hours (need 5 mph wind or greater).

18 Internet Sources Miami-South Florida National Weather Service (local weather forecasts and warnings, video weather briefings, model data, forecaster reasoning, etc.) weather.gov/miami Climate Prediction Center (long range outlooks)

19 NWS Miami Web Site Click on second map to get point forecast. weather.gov/miami

20 NWS Miami Web Site You can also enter city/state/zip at top left Caution: may not point to right area. Always confirm with map on right weather.gov/miami

21 NWS Miami Web Site Forecast Discussion provides meteorologist thinking behind forecast Jump to Radar & Satellite Images by clicking on corresponding images Hourly Weather Graph gives hour-by-hour detailed forecast for that location weather.gov/miami Forecast Database gives a graphical representation of forecast conditions.

22 VERY IMPORTANT Notes on NWS Temperature Forecasts NWS forecast temps are for standard height of 6 feet above ground with good exposure. Temps near ground (2 ft or below) are frequently a few to several degrees colder, especially on nights with light winds. Be familiar with your particular area and lay of the land. Temperatures in the field are usually several degrees colder than at reporting stations.

23 Monitoring Local Temperatures DO NOT RELY ON NWS SITES ALONE! Mesowest: Weather Underground/Wundermap: MADIS: IFAS Weather Stations:

24 NWS Sites Only

25 NWS + Supplemental Sites

26 Weather Underground

27 MADIS

28 IFAS Weather Stations

29 NOAA Climate Prediction Center: Good for long-range planning 6-10 day and 8-14 day outlooks give indication of above/below normal temperatures for that time frame. Updated each afternoon Interactive Display link gives additional detail

30 Timing of Temperatures/Winds First night after front usually the windiest night but NOT the coldest night. Depends on exact timing of frontal passage. Second or third night usually the coldest (with lighter winds but not totally calm the entire night). Overnight winds can be very fickle and tricky to forecast. Winds can keep temps up, but a brief calm period can drop temperatures several degrees in a very short time. Normally occurs second or third night.

31 Typical First Night temperature pattern. Steady temperature drop through the night with rather even temperature distribution.

32 Typical Second or Third Night temperature pattern. Uneven temperature drop with large difference between inland and metro/coast.

33 Timing of Temperatures/Winds Low dewpoints/dry air during afternoon leads to rapid temperature drop after sunset and increases freeze potential. Watch for very low relative humidity values during the late morning/early afternoon (below 35 percent) coupled with temperatures struggling to reach 60F despite full sun. Freezing temperatures can occur as early as Midnight, then steady or rising afterward. Official forecast graphs have very difficult time depicting the fast temperature drop

34 Temp/Wind Graphs for South Dade Agricultural Stations: Jan 3/4, 2012

35 Temperature increase between Midnight and 1 AM, fairly steady thereafter. Sapp Farm Station 2.5 SW of city of Homestead 25.46, Lull in wind around midnight followed by increase in wind. This corresponded with lowest temp around midnight and slow rise thereafter

36 Two minima, followed by steady to gradual increase after 1 AM. Accursio Station 4 SW of KTMB 25.61, Two minima corresponded with lulls in wind, then followed by increase after 1 AM.

37 Minimum around 1 AM followed by slow rise to steady temps thereafter. Alger s Farm Station 4 SW of KHST and east of Turnpike 25.46, Wind lull around 130 AM followed by increase.

38 Minimum around 1 AM. Homestead General Airport 5 NW of city of Homestead 25.5, Light winds through 1 AM, then slight increase afterwards.

39 Other Monkey Wrench High cloud cover which can sometimes be very hard to pinpoint.

40 NWS Forecasts Full forecast packages around 4-5 AM and 3-4 PM daily, with updates at intermediate times (9-10 AM and 8-9 PM)

41 Anticipating Freeze Events About 5-7 days out, forecast confidence can be quite low. Area forecast discussions are good to assess level of confidence: Also check Hazardous Weather Outlook: If trends hold, 3-5 days before an event a freeze outlook is issued. Continue to monitor forecast discussion and hazardous weather outlook.

42 Anticipating Freeze Events Continue to monitor up to day before event Forecast graphics give good areal perspective /sectors/mfl.php Point forecasts derived from these data

43 Final (VERY IMPORTANT) Note on NWS Temperature Forecasts NWS forecast temps are for standard height of 6 feet above ground with good exposure. Temps near ground (2 ft or below) are frequently a few to several degrees colder, especially on nights with light winds. Be familiar with your particular area and lay of the land. Temperatures in the field are usually several degrees colder than at reporting stations.

44 Questions? Thank You! Frost on car: NWS Miami parking lot Dec 28 th, 2010