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WHAT IS THE LAKE FORECAST MATRIX?

The Lake Forecast Matrix (LFM) displays forecasted weather parameters in 3 hour intervals out to 36 hours in the future.

HOW TO READ and INTERPRET THE LFM PRODUCT

There are several forecasted parameters which appear in the LFM product. Some of these values are forecasted in 12 hour intervals while others are forecasted in 3 hour intervals. Shown below is a partial example of a LFM product and a description of each of its parameters.

LAKE POINT FORECAST MATRICES
NATIONAL WEATHER SERVICE GRAY ME
120 PM EDT THU APR 25 2013

NHZ006-009-252115-
LAKE WINNIPESAUKEE - CENTER HARBOR
43.67N  71.42W ELEV. 504 FT
120 PM EDT THU APR 25 2013

DATE             THU 04/25/13            FRI 04/26/13
EDT 3HRLY     05 08 11 14 17 20 23 02 05 08 11 14 17 20 23
UTC 3HRLY     09 12 15 18 21 00 03 06 09 12 15 18 21 00 03

WIND DIR                W  W  W  N NE NE NE NE NE  E  N  N
WIND SPD               23 19 14 13 11 13 14 12 10 11 28 28
WIND GUST              41 35 26 25    25 26 23       57 57
H2O TEMP               40 40 40 40 40 40 40 40 40 40 40 40
WAVES                   0  0  0  0  0  0  0  0  0  0  0  0
MAX/MIN                      61          31          60
TEMP                   61 60 50 41 35 32 39 54 60 59 51 45
DEWPT                  19 20 21 23 25 26 27 25 24 27 29 29
RH                     19 21 31 48 66 78 62 32 25 29 42 53
CLOUDS                 SC SC SC OV OV OV OV BK BK BK SC FW
POP 12HR                      5          60          90
QPF 12HR                      0           0        0.02
LIGHTNING               1  1  1  4  4  4  4  5  5  5  1  1
RAIN SHWRS                       L  L  L  L  D  D  D      
TSTMS                            L  L  L  L  D  D  D     
      

1. WIND DIR:
WIND DIR is a snapshot of the expected wind direction forecast to occur at the indicated hour, using the 8 points of a compass (i.e., N, NE, E, SE, S, SW, W, NW). If a calm wind is forecast, double zeros (00) will be listed in place of a wind direction. WIND DIR is located below the hour to which it refers. WIND DIR is available in 3-hour increments.

2. WIND SPD:
WIND SPD is a snapshot of the sustained wind speed in miles per hour (MPH) forecast to occur at the indicated hour. If a calm wind is forecast, double zeros (00) will be listed in place of a wind speed. WIND SPD is valid in 3-hour increments.

2A. WIND GUST:
A wind gust row will appear in the 3HRLY block whenever forecasted wind gusts exceed the sustained wind speed (WIND SPD) by at least 10 MPH. WIND GUST is a snapshot valid on the hour indicated at the top of the corresponding column. WIND GUST is a snapshot of gusts of wind occurring at the indicated hour and is valid in 3-hour increments.

3. H2O TEMP:
H2O TEMP is a snapshot of the expected watertemperature in degrees F valid at the indicated hour. The water temperature is right justified in the column below the hour to which it refers. H2O TEMP is forecast at 3-hour intervals.

4. MIN/MAX:
(Alternatively labeled as MAX/MIN for morning issuance) 
A forecast of minimum or maximum temperatures in degrees Fahrenheit (F). The MAX temperature is valid for the period 7:00 a.m. through 7:00 p.m. local standard time, and MIN is valid for 7:00 p.m. through 8:00 a.m. local standard time (note that due to a 3-hour minimum time resolution, this element is right justified in the column beneath the approximate ending time of the MAX/MIN period). The nighttime MIN and daytime MAX will be displayed as single integer (e.g., -2, 8, 53, 102)..

5. TEMP:
TEMP is a snapshot of the expected ambient air temperature in degrees F valid at the indicated hour. The ambient air temperature is right justified in the column below the hour to which it refers. TEMP is forecast at 3-hour intervals.

6. DEWPT:
DEWPT is a snapshot of the expected dew point temperature in degrees F for the same time periods as its corresponding temperature forecast. DEWPT is located directly below the temperature line.

7. RH:
The relative humidity (RH) is a snapshot of the expected RH for the same time periods as its corresponding temperature and dew point forecast. The RH row is located directly below the DEWPT row. RH is available at 3-hour increments.

8. CLOUDS:
The CLOUDS category provides a snapshot of sky coverage during the indicated hour. CLOUDS is divided into five category codes ranging from clear to overcast . Each code represents an equivalent percentage of sky cover in percent. CLOUDS parameter is included in 3-hour time intervals.

Sky Cover Code Sky Cover Expression Equivalent Percent Opaque Sky Cover
CL Clear 0% to 6%
FW Mostly Clear/Mostly Sunny 7% to 31%
SC Partly Cloudy/Partly Sunny 32% to 69%
BK Mostly Cloudy 70% to 94%
OV Cloudy 95% to 100%

9. POP 12HR:
Probability of Precipitation (POP), is defined as the likelihood, expressed as a percent, of a measurable precipitation event (1/100th of an inch) at the given forecast point. The 12HR refers to the 12-hour valid time ending at 6:00 a.m. or 6:00 p.m. local time (0600 or 1800). The POP 12HR value is right justified in the column beneath the hour defining the ending time of the valid period.

10. QPF 12HR:
This parameter, quantitative precipitation forecast (QPF) represents the total amount of liquid precipitation, in inches, expected during a 12-hour period ending at 6:00 a.m., or 6:00 p.m. local time at the forecast point. The QPF is presented in single values. The QPF 12HR value is right justified in the column beneath the hour defining the ending time of the expected precipitation.

11. LIGHTNING:
This parameter, Lightning Activity Level, is a scale which describes frequency and character of cloud-to-ground lightning. Values are labeled 1-6:

Code Lightning Activity Level
1 No thunderstorms
2 Isolated thunderstorms. Light rain will occasionally reach the ground. Lightning is very infrequent, 1 to 5 cloud to ground strikes in a five minute period.
3 Widely scattered thunderstorms. Light to moderate rain will reach the ground. Lightning is infrequent, 6 to 10 cloud to ground strikes in a 5 minute period.
4 Scattered thunderstorms. Moderate rain is commonly produced Lightning is frequent, 11 to 15 cloud to ground strikes in a 5 minute period
5 Numerous thunderstorms. Rainfall is moderate to heavy. Lightning is frequent and intense, greater then 15 cloud to ground strikes in a 5 minute period.
6 Dry lightning (same as LAL 3 but without rain). This type of lightning has the potential for extreme fire activity and is normally highlighted in fire weather forecasts with a Red Flag Warning.

12, 13, and 14. PRECIPITATION TYPE AND CATEGORY:
The LFM may list several types of precipitation. Precipitation types only appear in the LFM if they are forecast to occur during the 36 hour forecast. Precipitation type codes are listed in the far left column of the LFM. For each type of precipitation that is forecast, an associated POP. category is specified within the body of the product. The POP. expression for the indicated precipitation type is valid at the hour specified by the column header. Precipitation type expressions are forecast at three hour time intervals.

Code Qualifying Term POP (%)
S Slight Chance 15% to 24%
C Chance 25% to 54%
L Likely 55% to 74%
O Occasional/Periods of 75% to 100%
D None 75% to 100%

Code Areal Term Areal Coverage (%)
IS Isolated 15% to 24%
SC Scattered 25% to 54%
NM Numerous 55% to 74%
EC None (extensive coverage) 75% to 100%

Code Areal Term Areal Coverage (%)
PA Patchy 1% to 24%
AR Areas 25% to 54%
WD Widespread 55% to 74%

15. OBVIS:
If an obstruction to visibility (OBVIS) is forecasted, a row labeled OBVIS will be listed underneath any forecast of precipitation. If no precipitation is forecast, then OBVIS will be listed under the row labeled CLOUDS. OBVIS is forecast at 3-hour intervals.

Code Obstruction to Visibility
F Fog
PF Patchy Fog
F+ Dense Fog
PF+ Patchy Dense Fog
H Haze
BS Blowing Snow
H Haze
BS Blowing Snow
K Smoke
BD Blowing Dust
VA Volcanic Ash

16. HEAT INDEX:
Heat Index is included based upon locally defined criteria.

 

National Weather Service
Gray Weather Forecast Office
P.O. Box 1208
1 Weather Lane, Route 231
Gray, ME 04039
207-688-3216
recorded forecast 207-688-3210 or
603-225-5191
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