2001 Helena, Montana (HLN)

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2001 HELENA, MONTANA (HLN)

Helena is located on the south side of an intermountain valley bounded on the west and south by the main chain of the Continental Divide. The valley is approximately 25 miles in width from north to south and 35 miles long from east to west. The average height of the mountains above the valley floor is about 3,000 feet.

The climate of Helena may be described as modified continental. Several factors enter into modifying the continental climate characteristics. Some of these are invasion by Pacific Ocean air masses, drainage of cool air into the valley from the surrounding mountains, and the protecting mountain shield in all directions.

The mountains to the north and east sometimes deflect shallow masses of invading cold Arctic air to the east. Following periods of extreme cold, when the return circulation of maritime air has brought warming to most of the eastern part of the state, cold air may remain trapped in the valley for several days before being replaced by warmer air. During these periods of transition from cold–to–warm temperatures, inversions are often quite pronounced.

As may be expected in a northern latitude, cold waves may occur from November through February, with temperatures occasionally dropping to zero or lower.

Summertime temperatures are moderate, with maximum readings generally under 90 degrees and very seldom reaching 100 degrees. Like all mountain stations, there is usually a marked change in temperature from day to night. During the summer this tends to produce an agreeable combination of fairly warm days and cool nights.

Most of the precipitation falls from April through July from frequent showers or thunderstorms, but usually with some steady rains in June, the wettest month of the year. Like summer, fall and winter months are relatively dry. During the April to September growing season, precipitation varies considerably.

Thunderstorms are rather frequent from May through August. Snow can be expected from September through May, but amounts during the spring and fall are usually light, and snow on the ground ordinarily lasts only a day or two. During the winter months snow may remain on the ground for several weeks at a time. There is little drifting of snow in the valley, and blizzard conditions are very infrequent.

Severe ice, sleet, and hailstorms are very seldom observed. Since 1880, only a few hailstorms have caused extensive damage in the city of Helena.

In winter, hours of sunshine are more than would be expected at a mountain location.

Due to the sheltering influence of the mountains, Foehn (Chinook) winds are not as pronounced as might be expected for a location on the eastern slopes of the Rocky Mountains. Strong winds can occur at any time throughout the year, but generally do not last more than a few hours at a time.

Based on the 1951–1980 period, the average first occurrence of 32 degrees Fahrenheit in the fall is September 18 and the average last occurrence in the spring is May 18.

NORMALS, MEANS, AND EXTREMES

HELENA, MT (HLN)

LATITUDE:    LONGITUDE:    ELEVATION (FT):    TIME ZONE:    WBAN: 24144

46 36′ 20″ N  111 57′ 49″ W  GRND: 3864  BARO: 3867  MOUNTAIN (UTC + 7)

ELEMENTPORJANFEBMARAPRMAYJUNJULAUGSEPOCTNOVDECYEAR
TEMPERATURE FNORMAL DAILY MAXIMUM3029.636.944.856.165.475.885.083.269.858.542.431.356.6
MEAN DAILY MAXIMUM5029.936.744.855.865.874.183.582.671.058.542.132.456.4
HIGHEST DAILY MAXIMUM61726977869310010210599877564105
YEAR OF OCCURRENCE200019951978199220011988198119691967200119991980AUG 1969
MEAN OF EXTREME MAXS.5452.355.464.275.383.690.795.895.588.877.761.952.974.5
NORMAL DAILY MINIMUM309.615.922.330.639.648.353.451.741.031.620.711.231.3
MEAN DAILY MINIMUM509.515.622.230.039.947.452.650.741.331.520.512.531.1
LOWEST DAILY MINIMUM61-42-42-3011730362818-8-39-38-42
YEAR OF OCCURRENCE195719961955195419541999197119921970199119591964FEB 1996
MEAN OF EXTREME MINS.54-18.6-9.3-.216.827.836.442.840.327.715.7-1.0-13.813.7
NORMAL DRY BULB3019.626.433.643.452.562.169.267.455.445.131.621.244.0
MEAN DRY BULB5419.025.833.043.252.660.668.066.656.045.131.422.043.6
MEAN WET BULB4916.722.527.335.643.650.254.553.446.037.527.519.536.2
MEAN DEW POINT499.615.319.225.934.341.644.542.836.929.220.613.227.8
NORMAL NO. DAYS WITH:
MAXIMUM 90300.00.00.00.00.22.47.66.61.10.00.00.017.9
MAXIMUM 323014.47.94.10.50.00.00.00.00.00.55.314.847.5
MINIMUM 323029.426.027.318.03.70.10.00.03.616.626.729.5180.9
MINIMUM 0308.94.51.70.00.00.00.00.00.00.11.66.223.0
H/CNORMAL HEATING DEG. DAYS30140710819736483881373465321617100213588031
NORMAL COOLING DEG. DAYS30000005016413933000386
RHNORMAL (PERCENT)3066646054545246485458656857
HOUR 05 LST3070727270727166667373737271
HOUR 11 LST3066625546454439414852626752
HOUR 17 LST3062544638383629303642576644
HOUR 23 LST3069686560605850526064697162
SPERCENT POSSIBLE SUNSHINE5547566159616478756860444260
W/OMEAN NO. DAYS WITH:
HEAVY FOG(VISBY 1/4 MI)611.91.21.80.30.20.10.10.10.10.51.42.310.0
THUNDERSTORMS610.10.10.11.04.17.48.67.51.80.30.10.031.1
CLOUDINESSMEAN:
SUNRISE-SUNSET (OKTAS)19.68.84.8
MIDNIGHT-MIDNIGHT (OKTAS)13.28.84.8
MEAN NO. DAYS WITH:
CLEAR11.52.02.08.02.011.07.01.06.0
PARTLY CLOUDY11.05.01.06.05.01.03.05.07.02.0
CLOUDY13.03.012.023.06.04.07.03.0
PRMEAN STATION PRESSURE(IN)2926.0226.0025.9425.9725.9625.9926.0426.0526.0726.0626.0226.0226.01
MEAN SEA-LEVEL PRES. (IN)5030.1730.1230.0329.9829.9529.9329.9629.9630.0330.0930.1230.1530.04
WINDSMEAN SPEED (MPH)389.09.410.711.011.212.012.011.710.59.89.99.610.6
PREVAIL. DIR (TENS OF DEGS)2127272727272727272727272727
MAXIMUM 2-MINUTE:
SPEED (MPH)746514741445247465251394452
DIR. (TENS OF DEGS)29262828291826262628272826
YEAR OF OCCURRENCE199719992001199719961999200119991999199919951995SEP 1999
MAXIMUM 5-SECOND:
SPEED (MPH)755645649556058596461475764
DIR. (TENS OF DEGS)28262827251926262626272926
YEAR OF OCCURRENCE200019992000199720011999200119991999199919951995SEP 1999
PRECIPITATIONNORMAL (IN)300.630.410.730.971.781.871.101.291.150.600.480.5911.60
MAXIMUM MONTHLY (IN)612.781.201.623.006.094.744.704.233.372.681.501.486.09
YEAR OF OCCURRENCE196919861982197519811944199319741965197519501977MAY 1981
MINIMUM MONTHLY (IN)61T0.020.020.100.290.080.080.020.080.020.040.01T
YEAR OF OCCURRENCE198719911959197719791985197319881972197819691997JAN 1987
MAXIMUM IN 24 HOURS (IN)610.770.581.011.252.311.782.261.861.611.010.820.512.31
YEAR OF OCCURRENCE196919531957195119811979198319741980200019591982MAY 1981
NORMAL NO. DAYS WITH:
PRECIPITATION 0.01308.26.58.68.111.110.17.17.67.15.26.98.294.7
PRECIPITATION 1.00300.00.00.00.00.20.20.10.10.10.00.00.00.7
SNOWFALLNORMAL (IN)309.15.67.45.51.70.1T0.01.62.25.08.546.7
MAXIMUM MONTHLY (IN)5735.619.721.620.612.72.7T6.213.711.032.922.835.6
YEAR OF OCCURRENCE196919591955196719671969199319921965196919591967JAN 1969
MAXIMUM IN 24 HOURS (IN)5711.511.78.712.912.52.7T6.213.37.421.510.721.5
YEAR OF OCCURRENCE196919931955196019671969199319921957196919591991NOV 1959
MAXIMUM SNOW DEPTH (IN)4623151588T00106191423
YEAR OF OCCURRENCE1969197519691986196719691965196919781985JAN 1969
NORMAL NO. DAYS WITH:
SNOWFALL 1.0303.01.82.31.50.40.*0.00.00.50.71.52.314.0
PRECIPITATION (inches) 2001 HELENA, MT (HLN)
YEARJANFEBMARAPRMAYJUNJULAUGSEPOCTNOVDECANNUAL
19721.120.540.630.410.771.120.561.630.080.570.330.468.22
19730.220.130.050.661.080.730.080.560.430.661.030.636.26
19740.660.230.380.762.070.340.494.230.220.510.300.2610.45
19751.260.720.883.001.952.833.892.470.472.680.480.3120.94
19760.260.380.411.340.872.740.291.581.820.040.300.0410.07
19770.650.131.110.101.821.371.370.721.930.170.481.4811.33
19780.960.610.310.941.200.442.830.591.110.021.190.7610.96
19790.770.721.342.260.292.750.320.790.120.380.060.5910.39
19800.620.740.880.634.323.161.920.282.571.210.320.4017.05
19810.150.101.100.756.091.151.780.100.900.820.540.3313.81
19820.800.581.620.541.772.990.490.742.740.350.311.0513.98
19830.240.070.360.291.792.203.482.671.560.350.260.7614.03
19840.170.150.491.451.032.140.111.110.730.740.470.419.00
19850.160.380.320.460.750.080.102.642.110.760.840.358.95
19860.321.200.491.080.831.561.371.842.450.030.540.3812.09
1987T0.031.190.761.901.502.880.380.800.050.120.4210.03
19880.270.500.451.321.821.500.360.022.090.690.690.3210.03
19891.420.821.350.721.001.431.551.611.310.540.260.4812.49
19900.470.140.910.431.540.920.402.570.110.110.360.478.43
19910.270.020.900.751.713.270.720.701.260.650.880.7911.92
19920.290.100.600.550.642.361.061.010.091.870.190.579.33
19930.801.030.561.631.713.144.702.791.250.710.360.1318.81
19940.200.400.321.451.230.840.710.470.091.140.550.077.47
19950.200.080.491.153.092.931.510.331.590.100.620.2812.37
19960.550.110.580.702.421.201.270.890.510.040.840.619.72
19970.280.100.100.202.352.431.251.790.311.620.130.0110.57
19980.490.120.390.642.273.032.960.500.820.141.070.1412.57
19990.380.260.021.052.192.150.411.920.540.390.130.109.54
20000.260.320.260.730.981.420.730.430.542.120.360.238.38
20010.270.170.441.391.232.111.940.431.380.540.130.2810.31
POR=120 YRS0.690.510.710.981.872.171.150.971.130.760.610.6112.16
AVERAGE TEMPERATURE (F) 2001 HELENA, MT (HLN)
YEARJANFEBMARAPRMAYJUNJULAUGSEPOCTNOVDECANNUAL
197212.426.839.139.850.961.861.766.050.939.031.113.641.1
197317.722.836.040.352.961.268.966.854.445.824.528.243.3
197418.132.833.946.048.664.970.461.553.545.834.727.744.8
197521.113.728.832.948.656.969.261.254.143.329.926.040.5
197624.929.530.443.254.156.667.664.857.944.933.828.744.7
197718.034.132.446.950.463.666.363.656.345.631.220.744.1
197816.922.838.847.753.263.267.265.557.946.522.715.243.1
19791.120.134.742.753.262.069.267.761.347.829.028.243.1
198014.325.331.349.055.459.867.362.956.845.434.926.944.1
198128.429.738.046.452.858.967.769.558.843.836.425.346.3
198216.523.733.940.550.961.268.669.155.544.927.422.742.9
198330.635.338.343.151.560.266.070.853.546.034.75.544.6
198427.332.437.243.552.860.070.069.852.341.733.111.644.3
198512.318.833.446.956.263.375.063.149.642.312.615.040.7
198625.521.842.943.353.666.464.268.051.245.329.018.644.2
198723.331.937.050.255.964.466.262.859.946.834.724.746.5
198818.829.136.147.155.568.471.368.656.450.333.823.146.5
198924.56.127.644.551.762.272.064.355.645.136.823.842.9
199028.826.934.945.851.061.569.468.463.645.337.414.045.6
199119.037.535.343.852.859.570.872.858.443.329.822.945.5
199223.734.242.747.757.564.964.364.756.746.732.814.745.9
199312.315.036.444.357.358.559.561.554.043.826.830.041.6
199429.820.739.445.656.262.769.069.961.644.328.323.645.9
199522.730.530.742.049.757.465.865.255.742.834.322.743.3
19969.921.125.748.261.368.366.655.543.823.116.6
199713.328.236.938.453.260.165.865.458.844.131.224.443.3
199821.231.032.844.853.955.169.868.661.943.834.524.545.2
199926.832.437.140.850.258.865.368.452.945.839.228.145.5
200025.028.838.647.054.962.472.169.656.444.622.016.244.8
200120.117.635.444.059.264.172.074.764.648.439.224.547.0
POR=121 YRS19.824.732.743.652.460.067.866.455.945.432.323.643.7

REFERENCE NOTES:

PAGE 1:

THE TEMPERATURE GRAPH SHOWS NORMAL MAXIMUM AND NORMAL MINIMUM DAILY TEMPERATURES (SOLID CURVES) AND THE ACTUAL DAILY HIGH AND LOW TEMPERATURES (VERTICAL BARS).

PAGE 2 AND 3:

H/C INDICATES HEATING AND COOLING DEGREE DAYS.

RH INDICATES RELATIVE HUMIDITY

W/O INDICATES WEATHER AND OBSTRUCTIONS

S INDICATES SUNSHINE.

PR INDICATES PRESSURE.

CLOUDINESS ON PAGE 3 IS THE SUM OF THE CEILOMETER AND SATELLITE DATA NOT TO EXCEED EIGHT EIGHTHS (OKTAS).

GENERAL:

T INDICATES TRACE PRECIPITATION, AN AMOUNT GREATER THAN ZERO BUT LESS THAN THE LOWEST REPORTABLE VALUE.

+ INDICATES THE VALUE ALSO OCCURS ON EARLIER DATES.

BLANK ENTRIES DENOTE MISSING OR UNREPORTED DATA.

NORMALS ARE 30–YEAR AVERAGES (1961–1990).

ASOS INDICATES AUTOMATED SURFACE OBSERVING SYSTEM.

PM INDICATES THE LAST DAY OF THE PREVIOUS MONTH.

POR (PERIOD OF RECORD) BEGINS WITH THE JANUARY DATA MONTH AND IS THE NUMBER OF YEARS USED TO COMPUTE THE MEAN. INDIVIDUAL MONTHS WITHIN THE POR MAY BE MISSING.

WHEN THE POR FOR A NORMAL IS LESS THAN 30 YEARS, THE NORMAL IS PROVISIONAL AND IS BASED ON THE NUMBER OF YEARS INDICATED.

0.* OR * INDICATES THE VALUE OR MEAN–DAYS–WITH IS BETWEEN 0.00 AND 0.05.

CLOUDINESS FOR ASOS STATIONS DIFFERS FROM THE NON–ASOS OBSERVATION TAKEN BY A HUMAN OBSERVER. ASOS STATION CLOUDINESS IS BASED ON TIME–AVERAGED CEILOMETER DATA FOR CLOUDS AT OR BELOW 12,000 FEET AND ON SATELLITE DATA FOR CLOUDS ABOVE 12,000 FEET.

THE NUMBER OF DAYS WITH CLEAR, PARTLY CLOUDY, AND CLOUDY CONDITIONS FOR ASOS STATIONS IS THE SUM OF THE CEILOMETER AND SATELLITE DATA FOR THE SUNRISE TO SUNSET PERIOD.

GENERAL CONTINUED:

CLEAR INDICATES 0–2 OKTAS, PARTLY CLOUDY INDICATES 3–6 OKTAS, AND CLOUDY INDICATES 7 OR 8 OKTAS. WHEN AT LEAST ONE OF THE ELEMENTS (CEILOMETER OR SATELLITE) IS MISSING, THE DAILY CLOUDINESS IS NOT COMPUTED.

WIND DIRECTION IS RECORDED IN TENS OF DEGREES (2 DIGITS) CLOCKWISE FROM TRUE NORTH. "00" INDICATES CALM. "36" INDICATES TRUE NORTH.

RESULTANT WIND IS THE VECTOR AVERAGE OF THE SPEED AND DIRECTION.

AVERAGE TEMPERATURE IS THE SUM OF THE MEAN DAILY MAXIMUM AND MINIMUM TEMPERATURE DIVIDED BY 2.

SNOWFALL DATA COMPRISE ALL FORMS OF FROZEN PRECIPITATION, INCLUDING HAIL.

A HEATING (COOLING) DEGREE DAY IS THE DIFFERENCE BETWEEN THE AVERAGE DAILY TEMPERATURE AND 65 F.

DRY BULB IS THE TEMPERATURE OF THE AMBIENT AIR.

DEW POINT IS THE TEMPERATURE TO WHICH THE AIR MUST BE COOLED TO ACHIEVE 100 PERCENT RELATIVE HUMIDITY.

WET BULB IS THE TEMPERATURE THE AIR WOULD HAVE IF THE MOISTURE CONTENT WAS INCREASED TO 100 PERCENT RELATIVE HUMIDITY.

ON JULY 1, 1996, THE NATIONAL WEATHER SERVICE BEGAN USING THE "METAR" OBSERVATION CODE THAT WAS ALREADY EMPLOYED BY MOST OTHER NATIONS OF THE WORLD. THE MOST NOTICEABLE DIFFERENCE IN THIS ANNUAL PUBLICATION WILL BE THE CHANGE IN UNITS FROM TENTHS TO EIGHTS (OKTAS) FOR REPORTING THE AMOUNT OF SKY COVER.

HEATING DEGREE DAYS (base 65 F) 2001 HELENA, MT (HLN)
YEARJULAUGSEPOCTNOVDECJANFEBMARAPRMAYJUNTOTAL
1972-731364941879810101588146511748927323741558791
1973-742347317588120811361452897956564500997787
1974-751613033858890511491355142911149545012358714
1975-7614119322666104511991236102310646493312577925
1976-771545219615928112014498621005535443907326
1977-7852922705931008136714851175806512361877808
1978-79326024456412631540197912509306653591289014
1979-801115127528107211381566114810394733041647585
1980-812581242602899117511279868325523711917083
1981-8221161956508531227149711539597264281367861
1982-8330163046181120130610598288236494171527322
1983-84760351584901184211649418566403801747909
1984-8527377716954165416251291973538266978500
1985-8631054556961571154512181202677645380428539
1986-876623409602107714321288923862437276777472
1987-8875104163556901124114261034889529297637278
1988-89101328244993412921251165011566104071078161
1989-900922746118391268111610589255734261777359
1990-91153178604823157914207679146303731597393
1991-9220220666105312971273884687513237946926
1992-93681412465649601552163113978796172282148497
1993-9417912232365211391080108012367835762681287566
1994-95181711063310951275130796310596814652227845
1995-9637612936839121303170312681210510127
1996-97155328665112511494159610268657933611528543
1997-9849531896421006125213529429906003362937704
1998-9928158650909124811769058597184511887272
1999-0079203545907671136123210438145353081196997
2000-01161327962512831504138513199126252121138286
2001-110665117661249
COOLING DEGREE DAYS (base 65 F) 2001 HELENA, MT (HLN)
YEARJANFEBMARAPRMAYJUNJULAUGSEPOCTNOVDECANNUAL
1972000023042890000163
197300004451511083000311
197400000102190310000323
1975000001154120000167
197600000141024510000171
197700000571015413000225
197800003371098537000271
1979000014515210321000322
198000001414104254000161
1981000001510916516000305
1982000003014715125000353
1983000041611518612000333
1984000010311651634000373
19850000255318540000429
198600003591451231000295
198700003661224115000247
19880000817021113230000551
19890000030222820000334
1990000007715914242000420
199100000218825029000469
199200001096551403000304
199300002241523000064
1994000016714617616000406
1995000005717324000173
19960000251251099000
19970000113817112000178
199800000016112472000357
19990000010931320000235
2000000025024516527000489
20010000419423730861000741
SNOWFALL (inches) 2001 HELENA, MT (HLN)
YEARJULAUGSEPOCTNOVDECJANFEBMARAPRMAYJUNTOTAL
1972-73T0.00.34.70.77.83.21.80.16.5T0.025.1
1973-740.00.01.37.212.57.29.95.92.21.50.20.047.9
1974-750.00.0T1.50.82.715.210.712.315.40.20.058.8
1975-760.00.00.06.34.93.93.34.46.710.90.00.040.4
1976-770.00.00.0T2.90.913.80.814.10.51.00.034.0
1977-780.00.00.00.46.819.515.713.32.60.9T0.059.2
1978-790.00.0TT22.113.811.77.212.49.3TT76.5
1979-800.00.00.00.20.66.59.310.510.13.10.00.040.3
1980-810.00.00.03.71.23.82.72.13.30.1T0.016.9
1981-820.00.00.05.23.46.118.44.813.94.10.80.056.7
1982-830.00.06.50.54.111.33.20.22.21.19.90.039.0
1983-840.00.06.40.01.313.01.31.55.72.30.80.032.3
1984-850.00.06.39.05.77.53.96.24.00.80.00.043.4
1985-860.00.02.98.810.48.54.215.61.211.60.20.063.4
1986-870.00.00.0T7.65.00.20.29.14.33.80.030.2
1987-880.00.00.00.30.91.24.48.05.22.90.00.022.9
1988-890.00.05.91.56.05.023.013.020.77.93.5T86.5
1989-900.0TT2.61.89.44.31.814.00.80.10.034.8
1990-910.00.50.00.58.611.65.90.812.25.60.40.546.6
1991-920.0T0.07.38.815.74.71.3T2.2T0.040.0
1992-93T6.2T8.61.511.810.217.92.32.5T0.261.2
1993-94T0.0T4.67.31.14.45.04.48.0T0.034.8
1994-950.0T0.0T8.62.00.31.68.10.00.0
1995-96T0.0T7.42.714.2
1996-97
1997-98
1998-99
1999-00T3.95.72.3
2000-01
2001-
POR=56 YRST0.11.52.36.38.28.66.07.34.81.40.146.6

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