Climatological Report (Monthly)
Issued by NWS Boise, ID

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CXUS55 KBOI 011824
CLMBOI

CLIMATE REPORT
NATIONAL WEATHER SERVICE BOISE ID
1123 AM MST MON JAN 1 2018

...................................

TEMPERATURE INVERSIONS PLAGUED THE TREASURE VALLEY MOST OF THE
MONTH, RESULTING IN BELOW NORMAL TEMPERATURES AT BOISE ON ALL BUT
EIGHT DAYS. ALTHOUGH IT RANKED AMONG THE COLDEST 25 PERCENT OF
DECEMBERS SINCE AIRPORT RECORDS BEGAN IN 1940, IT AVERAGED 5 DEGREES
WARMER THAN DECEMBER 2016.

A PERSISTENT HIGH PRESSURE RIDGE KEPT THE BOISE AREA DRY THROUGH
MUCH OF THE MONTH. PRECIPITATION WAS NEARLY HALF AN INCH BELOW
NORMAL, AND HALF THE DECEMBER 2016 TOTAL.

THE MONTH STARTED OUT FAIRLY MILD UNDER WEST-SOUTHWEST FLOW ALOFT
AHEAD OF AN UPPER LEVEL LOW PRESSURE TROUGH. THE TROUGH CROSSED THE
BOISE AREA ON THE 3RD, BRINGING THE LARGEST ONE-DAY PRECIPITATION OF
THE MONTH, MAINLY IN THE FORM OF RAIN.

AS THIS SYSTEM EXITED TO THE EAST, AN UPPER LEVEL HIGH PRESSURE
RIDGE BUILT OFFSHORE AND EXPANDED INLAND.

BY THE 5TH AN INVERSION HAD BECOME ESTABLISHED IN THE TREASURE
VALLEY, AND TEMPERATURES REMAINED BELOW FREEZING FROM THE 7TH
THROUGH THE 15TH.

ON THE 16TH AN UPPER LEVEL TROUGH DESTABILIZED THE ATMOSPHERE ENOUGH
TO BREAK THE INVERSION, AND TEMPERATURES CONTINUED TO WARM AS THE
UPPER LEVEL FLOW SHIFTED INTO THE WEST AND SOUTHWEST IN ADVANCE OF
YET ANOTHER UPPER LEVEL TROUGH. THE 19TH AND 20TH WERE THE WARMEST
DAYS OF THE MONTH, WITH HIGHS OF 52 AND 51.

BY THE 21ST THE TROUGH HAD MOVED EAST OF OUR AREA. IT WAS FOLLOWED
BY MOIST NORTHWEST FLOW ALOFT AND A COLD FRONT WHICH BROUGHT SNOW
TOTALING 6 INCHES ON THE 22ND AND 23RD. THE ACCUMULATION OF 5 INCHES
WAS THE FIRST MEASURABLE SNOW COVER SINCE FEBRUARY 4.

A COMBINATION OF MODIFIED ARCTIC AIR, OVERNIGHT CLEARING, FRESH SNOW
COVER, AND CALM WINDS CAUSED THE AIRPORT TEMPERATURE TO FALL TO 8
DEGREES BY SUNRISE ON THE 24TH, THE COLDEST READING SINCE THE 7
DEGREES ON JANUARY 18.

MORE SNOW WAS ON THE WAY. IT STARTED AT ABOUT 4 PM CHRISTMAS EVE AS
A WARM FRONT APPROACHED FROM THE WEST. BY CHRISTMAS MORNING 3 INCHES
OF NEW SNOW HAD FALLEN.

WARMING CONTINUED AT HIGHER ELEVATIONS, ENSURING THAT COLD AIR WOULD
REMAIN TRAPPED IN THE VALLEY FOR A FEW MORE DAYS.

ON THE 30TH A COLD FRONT PASSED THROUGH THE AREA. IT WAS FELT AS A
WARM FRONT IN THE VALLEY, AS COLDER AIR ALOFT BEHIND THE FRONT BROKE
THE INVERSION, ALLOWING THE AIRPORT TEMPERATURE TO CLIMB TO 43
DEGREES THAT AFTERNOON.

ON THE 31ST STRONG HIGH PRESSURE PROVIDED MOSTLY CLEAR SKIES.
ALTHOUGH A SHALLOW INVERSION HAD FORMED OVERNIGHT, THE SUN WARMED
THE SURFACE ENOUGH DURING THE DAY TO MIX THE AIR, AND TEMPERATURES
AROUND BOISE WARMED INTO THE 35-40 DEGREE RANGE.

...THE BOISE ID CLIMATE SUMMARY FOR THE MONTH OF DECEMBER 2017...

CLIMATE NORMAL PERIOD 1981 TO 2010
CLIMATE RECORD PERIOD 1875 TO 2017

WEATHER         OBSERVED          NORMAL  DEPART  LAST YEAR`S
                 VALUE   DATE(S)  VALUE   FROM    VALUE
                                          NORMAL
.............................................................
TEMPERATURE (F)
HIGHEST            52   12/19
LOWEST              8   12/24
AVG. MAXIMUM     34.7              37.5    -2.8
AVG. MINIMUM     22.5              24.0    -1.5
MEAN             28.6              30.7    -2.1
DAYS MAX >= 90      0
DAYS MAX <= 32     14
DAYS MIN <= 32     30
DAYS MIN <= 0       0

PRECIPITATION (INCHES)
RECORD
 MAXIMUM         5.96   1871
TOTALS           1.02              1.55   -0.53
DAYS >= .01         8
DAYS >= .10         5
DAYS >= .50         0
DAYS >= 1.00        0
GREATEST
 24 HR. TOTAL    0.35   12/03 TO 12/03

SNOWFALL (INCHES)
TOTALS            9.8
SINCE 7/1         9.8
GREATEST
 SNOW DEPTH         6   12/25
 24 HR TOTAL      4.0   12/22 TO 12/22

DEGREE_DAYS
HEATING TOTAL    1120              1061      59     1284
 SINCE 7/1       2344              2315      29
COOLING TOTAL       0                 0       0        0
 SINCE 1/1       1341               942     399
..............................................................

WIND (MPH)
AVERAGE WIND SPEED              5.3
RESULTANT WIND SPEED/DIRECTION   0/264
HIGHEST WIND SPEED/DIRECTION    35/300    DATE  12/20
HIGHEST GUST SPEED/DIRECTION    43/310    DATE  12/20

SKY COVER
POSSIBLE SUNSHINE (PERCENT)   MM

AVERAGE RH (PERCENT)     78


-  INDICATES NEGATIVE NUMBERS.
R  INDICATES RECORD WAS SET OR TIED.
MM INDICATES DATA IS MISSING.
T  INDICATES TRACE AMOUNT.

$$


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